Change checks of PyUnicode_Resize() return value for clarity.

Change checks of PyUnicode_Resize() return value for clarity.

The unicode_resize() family only returns -1 or 0 so simply checking
for != 0 is sufficient, but somewhat unclear.  Many Python API
functions return < 0 on error, reserving the right to return 0 or 1 on
success.  Change the call sites for consistency with these calls.