BLD: use unsigned instead of signed for lengths, avoid build warnings
I'm much more confident about the other two BLD PRs (#26757, #26758) than this, largely because:

- [ ] in a couple of comments here I wrote that a certain term should always be non-negative.  I haven't actually confirmed that this is always the case
- [ ] This changed things from int64_t to uint64_t, but could also reasonably have changed them to size_t.  I don't have a compelling reason for the correctness of the uint64_t choice.
# [Codecov](https://codecov.io/gh/pandas-dev/pandas/pull/26759?src=pr&el=h1) Report
> Merging [#26759](https://codecov.io/gh/pandas-dev/pandas/pull/26759?src=pr&el=desc) into [master](https://codecov.io/gh/pandas-dev/pandas/commit/0f3e8e8bd2118d56c7febe5d6081c5058cb22be7?src=pr&el=desc) will **decrease** coverage by `<.01%`.
> The diff coverage is `n/a`.

[![Impacted file tree graph](https://codecov.io/gh/pandas-dev/pandas/pull/26759/graphs/tree.svg?width=650&token=eZ4WkYLtcO&height=150&src=pr)](https://codecov.io/gh/pandas-dev/pandas/pull/26759?src=pr&el=tree)

```diff
@@            Coverage Diff             @@
##           master   #26759      +/-   ##
==========================================
- Coverage   91.71%    91.7%   -0.01%     
==========================================
  Files         178      178              
  Lines       50740    50740              
==========================================
- Hits        46538    46533       -5     
- Misses       4202     4207       +5
```

| Flag | Coverage Δ | |
|---|---|---|
| #multiple | `90.3% <ø> (ø)` | :arrow_up: |
| #single | `41.21% <ø> (-0.11%)` | :arrow_down: |

| [Impacted Files](https://codecov.io/gh/pandas-dev/pandas/pull/26759?src=pr&el=tree) | Coverage Δ | |
|---|---|---|
| [pandas/io/gbq.py](https://codecov.io/gh/pandas-dev/pandas/pull/26759/diff?src=pr&el=tree#diff-cGFuZGFzL2lvL2dicS5weQ==) | `78.94% <0%> (-10.53%)` | :arrow_down: |
| [pandas/core/frame.py](https://codecov.io/gh/pandas-dev/pandas/pull/26759/diff?src=pr&el=tree#diff-cGFuZGFzL2NvcmUvZnJhbWUucHk=) | `96.88% <0%> (-0.12%)` | :arrow_down: |
| [pandas/util/testing.py](https://codecov.io/gh/pandas-dev/pandas/pull/26759/diff?src=pr&el=tree#diff-cGFuZGFzL3V0aWwvdGVzdGluZy5weQ==) | `90.84% <0%> (-0.11%)` | :arrow_down: |

------

[Continue to review full report at Codecov](https://codecov.io/gh/pandas-dev/pandas/pull/26759?src=pr&el=continue).
> **Legend** - [Click here to learn more](https://docs.codecov.io/docs/codecov-delta)
> `Δ = absolute <relative> (impact)`, `ø = not affected`, `? = missing data`
> Powered by [Codecov](https://codecov.io/gh/pandas-dev/pandas/pull/26759?src=pr&el=footer). Last update [0f3e8e8...ac9536d](https://codecov.io/gh/pandas-dev/pandas/pull/26759?src=pr&el=lastupdated). Read the [comment docs](https://docs.codecov.io/docs/pull-request-comments).
 # [Codecov](https://codecov.io/gh/pandas-dev/pandas/pull/26759?src=pr&el=h1) Report
> Merging [#26759](https://codecov.io/gh/pandas-dev/pandas/pull/26759?src=pr&el=desc) into [master](https://codecov.io/gh/pandas-dev/pandas/commit/b9b081dc6b510c8290ded12fe751b1216843527e?src=pr&el=desc) will **decrease** coverage by `<.01%`.
> The diff coverage is `n/a`.

[![Impacted file tree graph](https://codecov.io/gh/pandas-dev/pandas/pull/26759/graphs/tree.svg?width=650&token=eZ4WkYLtcO&height=150&src=pr)](https://codecov.io/gh/pandas-dev/pandas/pull/26759?src=pr&el=tree)

```diff
@@            Coverage Diff             @@
##           master   #26759      +/-   ##
==========================================
- Coverage   91.97%   91.97%   -0.01%     
==========================================
  Files         180      180              
  Lines       50756    50756              
==========================================
- Hits        46685    46682       -3     
- Misses       4071     4074       +3
```

| Flag | Coverage Δ | |
|---|---|---|
| #multiple | `90.56% <ø> (ø)` | :arrow_up: |
| #single | `41.84% <ø> (-0.07%)` | :arrow_down: |

| [Impacted Files](https://codecov.io/gh/pandas-dev/pandas/pull/26759?src=pr&el=tree) | Coverage Δ | |
|---|---|---|
| [pandas/io/gbq.py](https://codecov.io/gh/pandas-dev/pandas/pull/26759/diff?src=pr&el=tree#diff-cGFuZGFzL2lvL2dicS5weQ==) | `88.88% <0%> (-11.12%)` | :arrow_down: |
| [pandas/core/frame.py](https://codecov.io/gh/pandas-dev/pandas/pull/26759/diff?src=pr&el=tree#diff-cGFuZGFzL2NvcmUvZnJhbWUucHk=) | `96.89% <0%> (-0.12%)` | :arrow_down: |
| [pandas/util/testing.py](https://codecov.io/gh/pandas-dev/pandas/pull/26759/diff?src=pr&el=tree#diff-cGFuZGFzL3V0aWwvdGVzdGluZy5weQ==) | `90.94% <0%> (+0.1%)` | :arrow_up: |

------

[Continue to review full report at Codecov](https://codecov.io/gh/pandas-dev/pandas/pull/26759?src=pr&el=continue).
> **Legend** - [Click here to learn more](https://docs.codecov.io/docs/codecov-delta)
> `Δ = absolute <relative> (impact)`, `ø = not affected`, `? = missing data`
> Powered by [Codecov](https://codecov.io/gh/pandas-dev/pandas/pull/26759?src=pr&el=footer). Last update [b9b081d...fa66545](https://codecov.io/gh/pandas-dev/pandas/pull/26759?src=pr&el=lastupdated). Read the [comment docs](https://docs.codecov.io/docs/pull-request-comments).
 OK by me. Does this offer any performance improvements? Wondering if Cython optimizes for not doing a wraparound check like in this SO question:

https://stackoverflow.com/a/37933120/621736 small comment, merge master and ping on green. Ping Thanks @jbrockmendel !