worktree: reject empty string

`git worktree add ""` errors out with the message `BUG: How come '' becomes
empty after sanitization?`, but not due to a bug in the  sanitization
code. An empty string should remain empty during sanitization. Instead reject
the argument as invalid user input, if it's already empty before sanitization.

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

Author: rimrul

builtin/worktree.c

@@ -496,6 +496,8 @@ static int add_worktree(const char *path, const char *refname,
 		die(_("invalid reference: %s"), refname);
 
 	name = worktree_basename(path, &len);
+	if (!len)
+		die(_("the empty string is not a valid worktree"));
 	strbuf_add(&sb, name, path + len - name);
 	sanitize_refname_component(sb.buf, &sb_name);
 	if (!sb_name.len)