Return an error ig scan results can not be parsed (fixes a segfault).

src/mm-generic-gsm.c

@@ -800,7 +800,9 @@ scan_done (MMSerial *serial,
         mm_callback_info_set_data (info, "scan-results", results, destroy_scan_data);
         g_match_info_free (match_info);
         g_regex_unref (r);
-    }
+    } else
+        info->error = g_error_new_literal (MM_MODEM_ERROR, MM_MODEM_ERROR_GENERAL,
+                                           "Could not parse scan results.");
 
  out:
     mm_callback_info_schedule (info);