gsocketclient: Handle cancellation between CONNECTING and CONNECTED

If a g_socket_client_connect_async() operation is cancelled between the
CONNECTING and CONNECTED events (i.e. while in the
g_socket_connection_connect_async() call), the code in
g_socket_client_connected_callback() would previously unconditionally
loop round and try the next socket address from the address enumerator
(by calling enumerator_next_async()). This would correctly handle the
cancellation and return from the overall task — but not before emitting
a spurious RESOLVING event.

Avoid emitting the spurious RESOLVING event by explicitly handling
cancellation at the beginning of g_socket_client_connected_callback().

https://bugzilla.gnome.org/show_bug.cgi?id=735179