Prevent an OOB read in `expand_dims.cc`

The for loop that follows this check assumes that `axis` is between `0` and `input_dims.size`. If user supplied `axis` is negative, the if code before this check is supposed to bring it back to positive (similar to how in Python one can do `l[-3]` to mean `l[-3 + len(l)]`).

PiperOrigin-RevId: 387200206
Change-Id: I162f4feba12d547c3a4340833ae682016a2ebfab