btrfs-progs: don't leak buffer on add_file_items() error

add_file_items() leaked "buffer" on this error return.
Free it first.

Resolves-Coverity-CID: 1125937
Signed-off-by: Eric Sandeen <sandeen@redhat.com>
Signed-off-by: Chris Mason <chris.mason@fusionio.com>

mkfs.c

@@ -629,6 +629,7 @@ static int add_file_items(struct btrfs_trans_handle *trans,
 		ret_read = pread64(fd, buffer, st->st_size, bytes_read);
 		if (ret_read == -1) {
 			fprintf(stderr, "%s read failed\n", path_name);
+			free(buffer);
 			goto end;
 		}