broadband-modem-mtk-legacy: fix GError memory leak

If the error matches, then a `+CIMI` command is sent to the device and
processing continues. However, the current error (for the `+CPIN?` command)
is not freed or propagated, fix that.

Fixes: 5c1fcbfe4a ("mtk-legacy: read IMSI if CPIN response is an error")
Signed-off-by: Barnabás Pőcze <pobrn@protonmail.com>

src/plugins/mtk/mm-broadband-modem-mtk-legacy.c

@@ -97,7 +97,7 @@ cpin_query_ready (MMIfaceModem *self,
 {
     MMModemLock  lock = MM_MODEM_LOCK_UNKNOWN;
     const gchar *result;
-    GError      *error = NULL;
+    g_autoptr (GError) error = NULL;
 
     result = mm_base_modem_at_command_finish (MM_BASE_MODEM (self), res, &error);
     if (error) {
@@ -118,7 +118,7 @@ cpin_query_ready (MMIfaceModem *self,
         }
 
         /* Otherwise just return the error */
-        g_task_return_error (task, error);
+        g_task_return_error (task, g_steal_pointer (&error));
     } else {
         if (result)
             lock = mm_parse_cpin_response (result, TRUE);