broadband-modem-mbim: fix segfault when loading capabilities

If loading capabilities using QMI over MBIM returns NONE without an
explicit error, the process would crash. Fix that by making the error
optional when NONE is received.

Fixes https://gitlab.freedesktop.org/mobile-broadband/ModemManager/issues/185

src/mm-broadband-modem-mbim.c

@@ -365,7 +365,7 @@ qmi_load_current_capabilities_ready (MMIfaceModem *self,
     ctx = g_task_get_task_data (task);
 
     ctx->current_qmi = mm_shared_qmi_load_current_capabilities_finish (self, res, &error);
-    if (!ctx->current_qmi) {
+    if (error) {
         mm_dbg ("Couldn't load currrent capabilities using QMI over MBIM: %s", error->message);
         g_clear_error (&error);
     }