When using tableam ExecFetchSlotHeapTuple() might return a separately
allocated tuple. We could use the shouldFree argument to explicitly
free it, but it seems more robust to to protect

Also add a CHECK_FOR_INTERRUPTS() after each tuple. It's likely that
each AM has (heap does) a CFI somewhere in the relevant path, but it
seems more robust to have one in validateForeignKeyConstraint()
itself.

Note that this only affects the cases that couldn't be optimized to be
verified with a query.

Author: Andres Freund
Reviewed-By: Tom Lane (in an earlier version)
Discussion:
    https://postgr.es/m/19030.1554574075@sss.pgh.pa.us
    https://postgr.es/m/CAKJS1f_SHKcPYMsi39An5aUjhAcEMZb6Cx1Sj1QWEWSiKJkBVQ@mail.gmail.com
    https://postgr.es/m/20180711185628.mrvl46bjgk2uxoki@alap3.anarazel.de