core:namespace: count and free failed paths inside chase_all_symlinks() (#4619)

This certainly fixes a bug that was introduced by PR
https://github.com/systemd/systemd/pull/4594 that intended to fix
https://github.com/systemd/systemd/issues/4567.

The fix was not complete. This patch makes sure that we count and free
all paths that fail inside chase_all_symlinks().

Fixes https://github.com/systemd/systemd/issues/4567