Fix type of local variables receiving result of PHT_HASH

Fix type of local variables receiving result of PHT_HASH

The variables should be of word type, not int or size_t.

* os_dep.c (GC_write_fault_handler, GC_remove_protection): Change type
of index local variable from size_t to word.
* os_dep.c [MPROTECT_VDB && DARWIN] (catch_exception_raise): Change
type of index local variable from int to word.