f_fastboot: Avoid use-after-free in the global pointer variable

In case of usb_add_function() failure the error path has an issue,
i.e the global pointer variable is assigned to garbage

Fix the above mentioned issue by assigning pointer to NULL.

Signed-off-by: Andy Shevchenko <andriy.shevchenko@linux.intel.com>

drivers/usb/gadget/f_fastboot.c

@@ -315,7 +315,7 @@ static int fastboot_add(struct usb_configuration *c)
 	status = usb_add_function(c, &f_fb->usb_function);
 	if (status) {
 		free(f_fb);
-		fastboot_func = f_fb;
+		fastboot_func = NULL;
 	}
 
 	return status;