use the malloc macros in pg_dump.c

Hi.

I found that pg_dump.c has many pg_mallocs where the new object/array
macros could have been used, but currently they are not. I'm not sure
if this was a deliberate or accidental omission.

In case it was accidental, here is a patch to modify/simplify all that
allocation code.

PSA v1.

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Kind Regards,
Peter Smith.
Fujitsu Australia