f2fs: avoid null dereference in f2fs_acl_from_disk

This patch resolves Coverity #751303:

>>> CID 753103: Explicit null dereferenced (FORWARD_NULL) Passing null
>>> pointer "value" to function "f2fs_acl_from_disk(char const *, size_t)",
	which dereferences it.

[Error path]
- value = NULL;
- retval = 0 by f2fs_getxattr();
- f2fs_acl_from_disk(value:NULL, ...);
Signed-off-by: Jaegeuk Kim <jaegeuk.kim@samsung.com>

fs/f2fs/acl.c

@@ -191,15 +191,14 @@ struct posix_acl *f2fs_get_acl(struct inode *inode, int type)
 		retval = f2fs_getxattr(inode, name_index, "", value, retval);
 	}
 
-	if (retval < 0) {
-		if (retval == -ENODATA)
+	if (retval > 0)
+		acl = f2fs_acl_from_disk(value, retval);
+	else if (retval == -ENODATA)
 		acl = NULL;
 	else
 		acl = ERR_PTR(retval);
-	} else {
-		acl = f2fs_acl_from_disk(value, retval);
-	}
 	kfree(value);
+
 	if (!IS_ERR(acl))
 		set_cached_acl(inode, type, acl);