wayland-util: return 0 on OOM in wl_map_insert_new()

If we cannot increase the array for new entries, we now return 0 instead
of accessing invalid memory.

krh: Edited to return 0 on failure instead.  In the initialization path,
we call wl_map_insert_new() to insert NULL at index 0, which also returns
0 but not as an error.  Since we do that up front, every other case of
returning 0 is an unambiguous error.

Signed-off-by: David Herrmann <dh.herrmann@googlemail.com>