MINOR: mworker: sets used or closed worker FDs to -1

MINOR: mworker: sets used or closed worker FDs to -1

mworker_cli_sockpair_new() is used to create the socketpair CLI listener of
the worker. Its FD is referenced in the mworker_proc structure, however,
once it's assigned to the listener the reference should be removed so we
don't use it accidentally.

The same must be done in case of errors if the FDs were already closed.