Suppress spurious Coverity warning in prune freeze logic

Suppress spurious Coverity warning in prune freeze logic

Adjust the prune_freeze_setup() parameter types of new_relfrozen_xid and
new_relmin_mxid to prevent misleading Coverity analysis.
heap_page_prune_and_freeze() compared these values against NULL when
passing them to prune_freeze_setup(), causing Coverity to assume they
could be NULL and flag a possible null-pointer dereference later, even
though it occurs inside a directly related conditional.

Reported-by: Coverity
Author: Melanie Plageman <melanieplageman@gmail.com>