read_gitfile_gently: fix use-after-free

The "dir" variable is a pointer into the "buf" array. When
we hit the cleanup_return path, the first thing we do is
free(buf); but one of the error messages prints "dir", which
will access the memory after the free.

We can fix this by reorganizing the error path a little. We
act on the fatal, error-printing conditions first, as they
want to access memory and do not care about freeing. Then we
free any memory, and finally return.
Signed-off-by: NJeff King <peff@peff.net>
Signed-off-by: NJunio C Hamano <gitster@pobox.com>

setup.c

@@ -479,19 +479,14 @@ const char *read_gitfile_gently(const char *path, int *return_error_code)
 	path = real_path(dir);
 
 cleanup_return:
-	free(buf);
-
 	if (return_error_code)
 		*return_error_code = error_code;
-
-	if (error_code) {
-		if (return_error_code)
-			return NULL;
-
+	else if (error_code) {
 		switch (error_code) {
 		case READ_GITFILE_ERR_STAT_FAILED:
 		case READ_GITFILE_ERR_NOT_A_FILE:
-			return NULL;
+			/* non-fatal; follow return path */
+			break;
 		case READ_GITFILE_ERR_OPEN_FAILED:
 			die_errno("Error opening '%s'", path);
 		case READ_GITFILE_ERR_TOO_LARGE:
@@ -509,7 +504,8 @@ const char *read_gitfile_gently(const char *path, int *return_error_code)
 		}
 	}
 
-	return path;
+	free(buf);
+	return error_code ? NULL : path;
 }
 
 static const char *setup_explicit_git_dir(const char *gitdirenv,