broadband-modem: ensure `match_info' is always initialized

This patch initializes `match_info' in registration_status_check_ready()
to NULL by default, such that `match_info' is always initialized even if
`self->priv->modem_3gpp_registration_regex' contains no elements.
Though `self->priv->modem_3gpp_registration_regex' always contains some
elements in the current implementation, it's better not to rely on that.

src/mm-broadband-modem.c

@@ -3952,7 +3952,7 @@ registration_status_check_ready (MMBroadbandModem *self,
     RunRegistrationChecksContext *ctx;
     const gchar *response;
     GError *error = NULL;
-    GMatchInfo *match_info;
+    GMatchInfo *match_info = NULL;
     guint i;
     gboolean parsed;
     gboolean cgreg;