worktree: don't read out of bounds

`worktree_basename` tries to read from memory before the passed `path`
string, if `path` is empty (or only consists of directory separators).
That results in unexpected nonsense data being returned to the caller,
which can lead to issues, such as `git worktree add ""` recursively
deleting the current working directory, including `.git`.

Stop reading out of bounds in these cases to avoid that behaviour.

This leads to `git worktree add ""` consistently exiting with the
message `BUG: How come '' becomes empty after sanitization?`, which is
still undesirable, but at least it doesn't result in data loss anymore.

This fixes git-for-windows#6346

Signed-off-by: Matthias Aßhauer <mha1993@live.de>

Author: rimrul

builtin/worktree.c

@@ -297,17 +297,21 @@ static void remove_junk_on_signal(int signo)
 static const char *worktree_basename(const char *path, int *olen)
 {
 	const char *name;
-	int len;
+	int len, len2;
 
-	len = strlen(path);
+	len2 = len = strlen(path);
 	while (len && is_dir_sep(path[len - 1]))
 		len--;
 
-	for (name = path + len - 1; name > path; name--)
-		if (is_dir_sep(*name)) {
-			name++;
-			break;
-		}
+	if(len) {
+		for (name = path + len - 1; name > path; name--)
+			if (is_dir_sep(*name)) {
+				name++;
+				break;
+			}
+	}
+	else
+		name = path + len2;
 
 	*olen = len;
 	return name;