pg_cryptohash_final possible out-of-bounds access (per Coverity)

Hi Hackers,

Per Coverity.

Coverity complaints about pg_cryptohash_final function.
And I agree with Coverity, it's a bad design.
Its allows this:

#define MY_RESULT_LENGTH 32

function pgtest(char * buffer, char * text) {
pg_cryptohash_ctx *ctx;
uint8 digest[MY_RESULT_LENGTH];

ctx = pg_cryptohash_create(PG_SHA512);
pg_cryptohash_init(ctx);
pg_cryptohash_update(ctx, (uint8 *) buffer, text);
pg_cryptohash_final(ctx, digest); // <--  CID 1446240 (#1 of 1):
Out-of-bounds access (OVERRUN)
pg_cryptohash_free(ctx);
return
}

Attached has a patch with suggestions to make things better.

regards,
Ranier Vilela