xprops: Fix reading Window and XSyncCounter properties

Both Window and XSyncCounter are XIDs which on 64 bit X clients are 8
bytes wide. But the values on the wire are 32 bit so, for these types,
we always copy 4 bytes into results->prop. As such copying them out
with a cast such as *(Window *) means that we are actually reading 8
bytes which depending on whether the higher addressed 4 bytes are zero
means that sometimes this works while others it gives us a bogus
value.

https://bugzilla.gnome.org/show_bug.cgi?id=756074