During repeated rescan of GiST index it's possible that scan key

During repeated rescan of GiST index it's possible that scan key
is NULL but SK_SEARCHNULL is not set. Add checking IS NULL of keys
to set during key initialization. If key is NULL and SK_SEARCHNULL is not
set then nothnig can be satisfied.
With assert-enabled compilation that causes coredump.

Bug was introduced in 8.3 by support of IS NULL index scan.