fs: NULL dereference in posix_acl_to_xattr()

This patch moves the dereference of "buffer" after the check for NULL.
The only place which passes a NULL parameter is gfs2_set_acl().

Cc: stable <stable@vger.kernel.org>
Signed-off-by: Dan Carpenter <dan.carpenter@oracle.com>
Signed-off-by: Steven Whitehouse <swhiteho@redhat.com>

fs/posix_acl.c

@@ -717,15 +717,16 @@ posix_acl_to_xattr(struct user_namespace *user_ns, const struct posix_acl *acl,
 		   void *buffer, size_t size)
 {
 	posix_acl_xattr_header *ext_acl = (posix_acl_xattr_header *)buffer;
-	posix_acl_xattr_entry *ext_entry = ext_acl->a_entries;
+	posix_acl_xattr_entry *ext_entry;
 	int real_size, n;
 
 	real_size = posix_acl_xattr_size(acl->a_count);
 	if (!buffer)
 		return real_size;
 	if (real_size > size)
 		return -ERANGE;
-
+
+	ext_entry = ext_acl->a_entries;
 	ext_acl->a_version = cpu_to_le32(POSIX_ACL_XATTR_VERSION);
 
 	for (n=0; n < acl->a_count; n++, ext_entry++) {