modules: Check current context before retrieving surrounding

There may be situations where this might get called while the
currently focused context just went away (eg. after setting the
text widget unsensitive).

Closes: #1317

modules/input/imwayland.c

@@ -121,6 +121,9 @@ notify_external_change (GtkIMContextWayland *context)
 {
   gboolean result;
 
+  if (!global->current)
+    return;
+
   context->surrounding_change = ZWP_TEXT_INPUT_V3_CHANGE_CAUSE_OTHER;
   g_signal_emit_by_name (global->current, "retrieve-surrounding", &result);
 }