BUG: Fix computation of invalid NaN indexes for interpolate.
Fixes issue #11115.

This change fixes up a couple edge cases for the computation of invalid NaN indexes for interpolation limits.

I've also added a test explicitly for the reported bug (and similar behaviors at the opposite end of the series) in #11115.

/cc @jreback 

in the whatsnew add this issue on (where the `limit_direction` change exists, just add this issue number on)
 Done!
 gr8. don't worry about eh failing builds on travis. fixing shortly
 thanks!
