Relocate a badly placed Assert in COPY FROM code

Relocate a badly placed Assert in COPY FROM code

There's not much point in asserting a pointer isn't NULL after some code
has already dereferenced that pointer.

Adjust the code so that the Assert occurs before the pointer dereference.

The Assert probably has questionable value in the first place, but it
seems worth keeping around to document the contract between
CopyMultiInsertInfoNextFreeSlot() and its callers.

Author: Amul Sul <sulamul@gmail.com>
Discussion: https://postgr.es/m/CAAJ_b94hXQzXaJxTLShkxQUgezf_SUxhzX9TH2f-g6gP7bne7g@mail.gmail.com