CheckNNConstraintFetch: Fill all of ConstrCheck in a single pass

CheckNNConstraintFetch: Fill all of ConstrCheck in a single pass

Previously, we'd fill all fields except ccbin, and only later obtain and
detoast ccbin, with hypothetical failures being possible.  If ccbin is
null (rare catalog corruption I have never witnessed) or its a corrupted
toast entry, we leak a tiny bit of memory in CacheMemoryContext from
having strdup'd the constraint name.  Repair these by only attempting to
fill the struct once ccbin has been detoasted.

Author: Ranier Vilela <ranier.vf@gmail.com>
Discussion: https://postgr.es/m/CAEudQAr=i3_Z4GvmediX900+sSySTeMkvuytYShhQqEwoGyvhA@mail.gmail.com