ublox: fix UAF in custom init ready URC handler

Clear the unsolicited message handler in ready_received() before
completing the task. Otherwise, if the modem sends subsequent READY
URCs, the handler would fire again using the freed GTask pointer.

https://gitlab.freedesktop.org/mobile-broadband/ModemManager/-/work_items/1074

Signed-off-by: Aleksander Morgado <aleksandermj@google.com>

src/plugins/ublox/mm-plugin-ublox.c

@@ -113,6 +113,9 @@ ready_received (MMPortSerialAt   *port,
     g_source_remove (ctx->timeout_id);
     ctx->timeout_id = 0;
 
+    mm_port_serial_at_add_unsolicited_msg_handler (port, ctx->ready_regex,
+                                                   NULL, NULL, NULL);
+
     mm_obj_dbg (probe, "received READY: port is AT");
 
     /* Flag as an AT port right away */