Doing so allows various crash possibilities. Fix by avoiding
having PrescanPreparedTransactions() increment
ShmemVariableCache->nextXid when it has no 2PC files

Bug found by Jeff Janes, diagnosis and patch by Pavan Deolasee,
then patch re-designed for clarity and full accuracy by
Michael Paquier.

Reported-by: Jeff Janes
Author: Pavan Deolasee, Michael Paquier
Discussion: https://postgr.es/m/CAMkU=1zMLnH_i1-PVQ-biZzvNx7VcuatriquEnh7HNk6K8Ss3Q@mail.gmail.com