unpack-file: fix ancient leak in create_temp_file()

Fix a leak that's been with us since 3407bb4940 (Add "unpack-file"
helper that unpacks a sha1 blob into a tmpfile., 2005-04-18). See
00c8fd493a (cat-file: use streaming API to print blobs, 2012-03-07)
for prior art which shows the same API pattern, i.e. free()-ing the
result of read_object_file() after it's used.

Signed-off-by: Ævar Arnfjörð Bjarmason <avarab@gmail.com>
Signed-off-by: Taylor Blau <me@ttaylorr.com>

builtin/unpack-file.c

@@ -19,6 +19,7 @@ static char *create_temp_file(struct object_id *oid)
 	if (write_in_full(fd, buf, size) < 0)
 		die_errno("unable to write temp-file");
 	close(fd);
+	free(buf);
 	return path;
 }