GtkIMContextIME: Avoid committing NULL string

We may get invalid UTF-16 sequences when third party
apps or hooks inject WM_CHAR messages.

Validate the unicode string before committing.

Fixes https://gitlab.com/inkscape/inkscape/-/issues/2088

modules/input/gtkimcontextime.c

@@ -336,6 +336,9 @@ gtk_im_context_ime_filter_keypress (GtkIMContext *context,
     return FALSE;
 
   utf8 = g_utf16_to_utf8 (event_priv->translation, event_priv->translation_len, NULL, NULL, NULL);
+  if (!utf8)
+    return FALSE;
+
   g_signal_emit_by_name (context_ime, "commit", utf8);
   g_free (utf8);