BUG: fix potentially infinite recursion in VOID_nonzero. Fixes #3312.

Commit https://github.com/numpy/numpy/commit/b7cc20ad3400f650bbc45feb5a0bad041722ef29 contains a typo; the code modifying the array sets `descr` to the saved value instead of the temporary value, giving recursive behaviour that only stops when `VOID_nonzero` walks off the array.