cifs: fix leak in FSCTL_ENUM_SNAPS response handling

The server may respond with success, and an output buffer less than
sizeof(struct smb_snapshot_array) in length. Do not leak the output
buffer in this case.

Fixes: 834170c8 ("Enable previous version support")
Signed-off-by: David Disseldorp <ddiss@suse.de>
CC: Stable <stable@vger.kernel.org>
Signed-off-by: Steve French <smfrench@gmail.com>