Fix thinkos from 4f4061b2 for libpq integer parsing

A check was redundant.  While on it, add an assertion to make sure that
the parsing routine is never called with a NULL input.  All the code
paths currently calling the parsing routine are careful with NULL inputs
already, but future callers may forget that.

Reported-by: Peter Eisentraut, Lars Kanis
Discussion: https://postgr.es/m/ec64956b-4597-56b6-c3db-457d15250fe4@2ndquadrant.com
Backpatch-through: 12