Fix calloc check if oom (PQcancelCreate)

[Ranier Vilela]

Hi.

I think that commit 61461a3
<https://github.com/postgres/postgres/commit/61461a300c1cb5d53955ecd792ad0ce75a104736>,
left some oversight.
The function *PQcancelCreate* fails in check,
return of *calloc* function.

Trivial fix is attached.

But, IMO, I think that has more problems.
If any allocation fails, all allocations must be cleared.
Or is the current behavior acceptable?

best regards,
Ranier Vilela

fix-calloc-check-oom-PQcancelCreate.patch
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