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/ui/core.c

@@ -712,6 +712,8 @@ ui_close_win(int index)
         }
     }
 
+    // remove the IQ handlers
+    iq_handlers_remove_win(window);
     wins_close_by_num(index);
     title_bar_console();
     status_bar_current(1);