usb: gadget: gserial: check if console kthread exists

Check for bad pointer that may result because of kthread_create failure.
This check is needed since the gserial setup callback function
(gs_console_setup()) is only freeing the info->con_buf in case of
kthread_create failure which will result into bad info->console_thread
pointer.
Without checking info->console_thread pointer validity in the
gserial_console_exit() function, before calling kthread_stop(), the
rmmod will generate Kernel Oops.

Signed-off-by: Bogdan Mirea <[e-mail]>
Signed-off-by: Felipe Balbi <[e-mail]>

drivers/usb/gadget/function/u_serial.c

···
 	struct gscons_info *info = &gscons_info;
 
 	unregister_console(&gserial_cons);
-	if (info->console_thread != NULL)
+	if (!IS_ERR_OR_NULL(info->console_thread))
 		kthread_stop(info->console_thread);
 	gs_buf_free(&info->con_buf);
 }