gst/gstbin.c: Can't use GPOINTER_TO_INT and GINT_TO_POINTER with GTypes.

gst/gstbin.c: Can't use GPOINTER_TO_INT and GINT_TO_POINTER with GTypes.

Original commit message from CVS:
* gst/gstbin.c: (compare_interface), (gst_bin_get_by_interface),
(gst_bin_iterate_all_by_interface):
Can't use GPOINTER_TO_INT and GINT_TO_POINTER with GTypes.
GTypes are gulongs and thus the top 4 bytes might be cut
off on some platforms when doing GPOINTER_TO_INT, leading
to invalid GTypes and bad things happening.
Also add a check to make sure the type passed in is really
an interface type.