usb: dbgp gadget: fix use after free in dbgp_unbind()

After dbgp_bind()-dbgp_unbind() cycle happens, static variable dbgp
contains pointers to already deallocated memory (dbgp.serial and dbgp.req).
If the next dbgp_bind() fails, for example in usb_ep_alloc_request(),
dbgp_bind() calls dbgp_unbind() on failure path,
and dbgp_unbind() frees dbgp.serial that still stores a pointer
to already deallocated memory.

The patch sets pointers to NULL in dbgp_unbind().

Found by Linux Driver Verification project (linuxtesting.org).
Signed-off-by: Alexey Khoroshilov <khoroshilov@ispras.ru>
Signed-off-by: Felipe Balbi <balbi@ti.com>

drivers/usb/gadget/legacy/dbgp.c

@@ -222,10 +222,12 @@ static void dbgp_unbind(struct usb_gadget *gadget)
 {
 #ifdef CONFIG_USB_G_DBGP_SERIAL
 	kfree(dbgp.serial);
+	dbgp.serial = NULL;
 #endif
 	if (dbgp.req) {
 		kfree(dbgp.req->buf);
 		usb_ep_free_request(gadget->ep0, dbgp.req);
+		dbgp.req = NULL;
 	}
 
 	gadget->ep0->driver_data = NULL;