cat-file: fix memory leak

Discovered by Coverity.

Signed-off-by: Johannes Schindelin <johannes.schindelin@gmx.de>

builtin/cat-file.c

@@ -165,6 +165,7 @@ static int cat_one_file(int opt, const char *exp_type, const char *obj_name,
 		die("git cat-file %s: bad file", obj_name);
 
 	write_or_die(1, buf, size);
+	free(buf);
 	return 0;
 }