USB: fix possible null deref in init_usb_class()

Add a missing goto.  We dereference usb_class on the next line.

Found by smatch static checker.

Signed-off-by: Dan Carpenter <error27@gmail.com>
Signed-off-by: Greg Kroah-Hartman <gregkh@suse.de>

diff --git a/drivers/usb/core/file.c b/drivers/usb/core/file.c
index 222ee07..bfc6c2e 100644
--- a/drivers/usb/core/file.c
+++ b/drivers/usb/core/file.c
@@ -99,6 +99,7 @@
 		printk(KERN_ERR "class_create failed for usb devices\n");
 		kfree(usb_class);
 		usb_class = NULL;
+		goto exit;
 	}
 	usb_class->class->devnode = usb_devnode;