write-tree: Avoid leak when index refers to an invalid object

Noticed by valgrind during test t0000.35 “writing this tree without
--missing-ok”.

Even in the cherry-pick foo..bar code path, such an error is the
end of the line.  But maybe some day an interactive porcelain will
want to link to libgit, making this matter.

Signed-off-by: Jonathan Nieder <jrnieder@gmail.com>
Signed-off-by: Junio C Hamano <gitster@pobox.com>

Author: jrn

cache-tree.c

@@ -328,9 +328,11 @@ static int update_one(struct cache_tree *it,
 			mode = ce->ce_mode;
 			entlen = pathlen - baselen;
 		}
-		if (mode != S_IFGITLINK && !missing_ok && !has_sha1_file(sha1))
+		if (mode != S_IFGITLINK && !missing_ok && !has_sha1_file(sha1)) {
+			strbuf_release(&buffer);
 			return error("invalid object %06o %s for '%.*s'",
 				mode, sha1_to_hex(sha1), entlen+baselen, path);
+		}
 
 		if (ce->ce_flags & CE_REMOVE)
 			continue; /* entry being removed */