Fix mem leak in read_csv
Pending validation but I believe this fixes #57039

Initially I misread this function was branching based off a return value that was always 0, but the actual value lies in the side effects
@akrherz do you have a quick way of checking this? @WillAyd I am sorry, I have never built pandas before, but will try it now. I certainly appreciate the help here. I was able to build pandas, whew.
main branch memray screenshot (showing RSS) with the reproducing leak script
![Screenshot from 2024-01-26 09-14-05](https://github.com/pandas-dev/pandas/assets/210858/454b9a0b-e676-4081-9f21-6830760b7b40)
running @WillAyd/fix-mem-leak branch
![Screenshot from 2024-01-26 09-19-34](https://github.com/pandas-dev/pandas/assets/210858/4e180ba8-304d-47c2-bae1-b1a2f52ab830)

LGTM! @WillAyd could you add a `peakmem` ASV benchmark for this case? I've added, but in this case it unfortunately would not have made a difference because the extension uses the system `free` call instead of `PyMem_Free`, so the Python interpreter does not track this.

Maybe we _should_ use PyMem_Free, although the downside to that would be that it doesn't play nicely with LSAN I don't wish to waste dev's time here, but I am curious why the introduction of `np.nan` in my code causes this memory leak to trigger?  A terse response of "its complicated" is more than sufficient :) An automated test would not have caught this memory leak, perhaps... Thanks @WillAyd  > I don't wish to waste dev's time here, but I am curious why the introduction of `np.nan` in my code causes this memory leak to trigger? A terse response of "its complicated" is more than sufficient :) An automated test would not have caught this memory leak, perhaps...

I think np.nan is a red herring - it is definitely a mistake in the code to get rid of those free blocks and LSAN detects the leak either way

Thanks again for the quick report! > I think np.nan is a red herring

Thank you, but the memory leak does not happen with that line removed.  Regardless, I am thrilled this is fixed :)