BUG: Avoid double iterator deallocation in `ufunc_object.c`

In `PyUFunc_GeneralizedFunction`, whenever a call to
`NpyIter_GetIterNext` fails, `NpyIter_Deallocate` is called twice, one
right after the error check, the second in the `goto fail`. Removed
the first one.
Merge pull request #4784 from jaimefrio/iter-dealloc-gufunc

BUG: Avoid double iterator deallocation in `ufunc_object.c`