composetable: Don't abort on unsupported escape sequences

People might put all sorts of gunk in their .XCompose file, in
the hope that XLib makes sense of it. Even if we don't make sense
of it, we shouldn't abort, but instead ignore the lines we can't
understand. Pointed out in
https://bugzilla.redhat.com/show_bug.cgi?id=1301254

gtk/gtkcomposetable.c

@@ -104,7 +104,9 @@ parse_compose_value (GtkComposeData *compose_data,
         uch = g_ascii_strtoll(words[1] + 1, NULL, 8);
       /* If we need to handle other escape sequences. */
       else if (uch != '\\')
-        g_assert_not_reached ();
+        {
+          g_warning ("Invalid escape sequence: %s: %s", val, line);
+        }
     }
 
   if (g_utf8_get_char (g_utf8_next_char (words[1])) > 0)