Don't do unsafe pointer casting

Don't cast a pointer-to-int into a pointer-to-pointer and pass the
resulting pointer to g_hash_table_lookup_extended() - pointers and ints
are *not* guaranteed to be the same size.  Instead, just have a variable
of type gpointer, pass a pointer to *that*, and then run that result
through GPOINTER_TO_UINT().

This fixes a reproducible crash.

Change-Id: I42954f222ab59866cb909b80d9dbb1d2668d2aff
Reviewed-on: https://code.wireshark.org/review/5457
Reviewed-by: Guy Harris <guy@alum.mit.edu>