Wrong buffer limits check

Hi hackers,

I have tried to analyse Postgres code with Svace static analyzer [1] and
found something I think is a real bug.

In pgp-decrypt.c, in prefix_init function the following check:
if (len > sizeof(tmpbuf))

seem to be erroneous and should really look this way:
if (len > PGP_MAX_BLOCK)

Otherwise the below checks in this line could lead to buffer overflows:
if (buf[len - 2] != buf[len] || buf[len - 1] != buf[len + 1])

This is because buf will point to tmpbuf, while tmpbuf have a size of
PGP_MAX_BLOCK + 2.

What do you think? The proposed patch towarts the current master branch is
attached.

[1] - https://svace.pages.ispras.ru/svace-website/en/

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    Mikhail A. Gribkov

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