Fix possible core dump in parallel restore when using a TOC list.

Commit 3eb9a5e7 unintentionally introduced an ordering dependency
into restore_toc_entries_prefork().  The existing coding of
reduce_dependencies() contains a check to skip moving a TOC entry
to the ready_list if it wasn't initially in the pending_list.
This used to suffice to prevent reduce_dependencies() from trying to
move anything into the ready_list during restore_toc_entries_prefork(),
because the pending_list stayed empty throughout that phase; but it no
longer does.  The problem doesn't manifest unless the TOC has been
reordered by SortTocFromFile, which is how I missed it in testing.

To fix, just add a test for ready_list == NULL, converting the call
with NULL from a poor man's sanity check into an explicit command
not to touch TOC items' list membership.  Clarify some of the comments
around this; in particular, note the primary purpose of the check for
pending_list membership, which is to ensure that we can't try to restore
the same item twice, in case a TOC list forces it to be restored before
its dependency count goes to zero.

Per report from Fabrízio de Royes Mello.  Back-patch to 9.3, like the
previous commit.

Discussion: https://postgr.es/m/CAFcNs+pjuv0JL_x4+=71TPUPjdLHOXA4YfT32myj_OrrZb4ohA@mail.gmail.com