Fix out-of-array-bounds compiler warning

Fix out-of-array-bounds compiler warning

Since the array length check is using a post-increment operator, the
compiler complains that there's a potential write to one element beyond
the end of the array.  This is not possible currently: the only path to
this function is through pg_get_object_address(), which already verifies
that the input array is no more than two elements in length.  Still, a
bug is a bug.

No idea why my compiler doesn't complain about this ...

Pointed out by Dead Rasheed and Peter Eisentraut