Only handle MAM response if window still exists

Before this patch the following scenario lead to a segfault:
1. open a window that sends a MAM request
2. fast enough close that window again before the MAM response was
   processed
Once the MAM response is received we'd call `_mam_rsm_id_handler()`
from the `_iq_handler()` and `window` would point to a non-existant window
which leads to a segfault.

Signed-off-by: Steffen Jaeckel <jaeckel-floss@eyet-services.de>

src/xmpp/xmpp.h

@@ -240,6 +240,7 @@ void iq_enable_carbons(void);
 void iq_disable_carbons(void);
 void iq_send_software_version(const char* const fulljid);
 void iq_rooms_cache_clear(void);
+void iq_handlers_remove_win(ProfWin* window);
 void iq_handlers_clear();
 void iq_room_list_request(gchar* conferencejid, gchar* filter);
 void iq_disco_info_request(gchar* jid);