builtin/mv.c: fix possible segfault in add_slash()

A possible segfault was introduced in c08830de41 (mv: check if
<destination> is a SKIP_WORKTREE_DIR, 2022-08-09).

When running t7001 with SANITIZE=address, problem appears when running:

	git mv path1/path2/ .
or
	git mv directory ../
or
	any <destination> that makes dest_path[0] an empty string.

The add_slash() call could segfault when path argument to it is an empty
string, because it makes an out-of-bounds read to decide if an extra
slash '/' needs to be appended to it.

As add_slash() is used to make sure that a valid pathname to a file in
the given directory can be made by appending a filename after the value
returned from it, if path is an empty string, we want to return it
as-is.  The path to a file "F" in the top-level of the working tree
(i.e. path=="") is formed by appending "F" after "" (i.e. path) without
any slash in between.

So, just like the case where a non-empty path already ends with a slash,
return an empty path as-is.

Reported-by: Jeff King <peff@peff.net>
Helped-by: Jeff King <peff@peff.net>
Helped-by: Junio C Hamano <gitster@pobox.com>
Helped-by: Derrick Stolee <derrickstolee@github.com>
Signed-off-by: Shaoxuan Yuan <shaoxuan.yuan02@gmail.com>
Signed-off-by: Junio C Hamano <gitster@pobox.com>

builtin/mv.c

@@ -71,7 +71,7 @@ static const char **internal_prefix_pathspec(const char *prefix,
 static const char *add_slash(const char *path)
 {
 	size_t len = strlen(path);
-	if (path[len - 1] != '/') {
+	if (len && path[len - 1] != '/') {
 		char *with_slash = xmalloc(st_add(len, 2));
 		memcpy(with_slash, path, len);
 		with_slash[len++] = '/';