[PATCH v1.5] worktree: Fix out of bounds read that causes data loss and reject invalid empty input in worktree add

["René Scharfe" <l.s.r@web.de>]

From: =?UTF-8?q?Matthias=20A=C3=9Fhauer?= <mha1993@live.de>

`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 https://github.com/git-for-windows/git/issues/6346

Signed-off-by: René Scharfe <l.s.r@web.de>
---
How about this while we're waiting for a reroll?  It implements what the
commit message says, nothing more.  Follows the style of the first loop.

 builtin/worktree.c | 8 +++-----
 1 file changed, 3 insertions(+), 5 deletions(-)

diff --git a/builtin/worktree.c b/builtin/worktree.c
index 654d27c3e1..a770dd5ead 100644
--- a/builtin/worktree.c
+++ b/builtin/worktree.c
@@ -303,11 +303,9 @@ static const char *worktree_basename(const char *path, int *olen)
 	while (len && is_dir_sep(path[len - 1]))
 		len--;
 
-	for (name = path + len - 1; name > path; name--)
-		if (is_dir_sep(*name)) {
-			name++;
-			break;
-		}
+	name = path + len;
+	while (name > path && !is_dir_sep(name[-1]))
+		name--;
 
 	*olen = len;
 	return name;
-- 
2.55.0