exec-invoke: don't double-close FDs on error

exec-invoke: don't double-close FDs on error

When a late error occurs in sd-executor, the cleanup-on-close of the
context structs happen, but at that time all FDs might have already
been closed via close_all_fds(), so a double-close happens. This
can be seen when DynamicUser is enabled, with a non-existing
WorkingDirectory.

Invalidate the FDs in the context structs if close_all_fds succeeds.