submodule_uses_worktrees(): plug memory leak

There is really no reason why we would need to hold onto the allocated
string longer than necessary.

Reported by Coverity.

Signed-off-by: Johannes Schindelin <johannes.schindelin@gmx.de>
Signed-off-by: Junio C Hamano <gitster@pobox.com>

worktree.c

@@ -399,6 +399,7 @@ int submodule_uses_worktrees(const char *path)
 
 	/* The env would be set for the superproject. */
 	get_common_dir_noenv(&sb, submodule_gitdir);
+	free(submodule_gitdir);
 
 	/*
 	 * The check below is only known to be good for repository format
@@ -418,7 +419,6 @@ int submodule_uses_worktrees(const char *path)
 	/* See if there is any file inside the worktrees directory. */
 	dir = opendir(sb.buf);
 	strbuf_release(&sb);
-	free(submodule_gitdir);
 
 	if (!dir)
 		return 0;