Added check if gitdir exists in is_prunable()

Fixes libgit2#5598

Author: rbmclean

src/libgit2/worktree.c

@@ -565,6 +565,7 @@ int git_worktree_is_prunable(git_worktree *wt,
 	git_worktree_prune_options *opts)
 {
 	git_worktree_prune_options popts = GIT_WORKTREE_PRUNE_OPTIONS_INIT;
+	git_buf path = GIT_BUF_INIT;
 
 	GIT_ERROR_CHECK_VERSION(
 		opts, GIT_WORKTREE_PRUNE_OPTIONS_VERSION,
@@ -595,6 +596,14 @@ int git_worktree_is_prunable(git_worktree *wt,
 		return 0;
 	}
 
+	if (git_buf_printf(&path, "%s/worktrees/%s", wt->commondir_path, wt->name) < 0)
+		return 0;
+	if (!git_path_exists(path.ptr))
+	{
+		git_error_set(GIT_ERROR_WORKTREE, "worktree gitdir '%s' does not exist", path.ptr);
+		return 0;
+	}
+
 	return 1;
 }