Fix thinko: have trueTriConsistentFn return GIN_TRUE.

While we're at it, also improve comments in ginlogic.c.
During index build, check and elog (not just Assert) for broken HOT chain.

The recently-fixed bug in WAL replay could result in not finding a parent
tuple for a heap-only tuple.  The existing code would either Assert or
generate an invalid index entry, neither of which is desirable.  Throw a
regular error instead.
Release notes for 9.3.4, 9.2.8, 9.1.13, 9.0.17, 8.4.21.