Avoid treating nonrequired nbtree keys as required.

Consistently prevent nbtree array advancement from treating a scankey as
required when operating in pstate.forcenonrequired mode.  Otherwise, we
risk a NULL pointer dereference.  This was possible in the path where
_bt_check_compare is called to recheck a tuple that advanced all of the
scan's arrays to matching values: its continuescan=false handling
expects _bt_advance_array_keys to have been called with a valid pstate,
but it'll always be NULL during sktrig_required=false calls (which is
how _bt_advance_array_keys must be called when pstate.forcenonrequired).

Oversight in commit 8a510275, which optimized nbtree search scan key
comparisons.

Author: Peter Geoghegan <pg@bowt.ie>
Reported-By: Mark Dilger <mark.dilger@enterprisedb.com>
Discussion: https://postgr.es/m/CAHgHdKsn2W=gPBmj7p6MjQFvxB+zZDBkwTSg0o3f5Hh8rkRrsA@mail.gmail.com
Discussion: https://postgr.es/m/CAH2-WzmodSE+gpTd1CRGU9ez8ytyyDS+Kns2r9NzgUp1s56kpw@mail.gmail.com

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