[PATCH] Improve treatment of page special and page header alignment during page init.

I was looking at changes in Sp-Gist by commit 4c0239cb7a7775e3183cb575e62703d71bf3302d

https://postgr.es/m/CALj2ACViOo2qyaPT7krWm4LRyRTw9kOXt+g6PfNmYuGA=YHj9A@mail.gmail.com ) and realized that during PageInit, both page header and page special are expected to be maxaligned but in reality, their treatment is quite different: 

1. page special size is silently enforced to be maxaligned by PageInit() even if caller-specified specialSize is not of a maxalign'ed size.

2. page header size alignment is not checked at all (but we expect it maxalign'ed, yes).

I'd propose do both things in the same way: just Assert both sizes are maxalign'ed during page init. 

I dived further and it appears that the only caller, who provides not properly aligned page special is fill_seq_with_data() and corrected it. 

I am really convinced, that _callers_ should care about proper special size. So now PageInit() just checks the right lengths of page special and page header with assert, not enforcing size change silently. PFA my small patch on this. I'd propose it to commit if in the HEAD only likewise the commit 4c0239cb7a7775e3183cb575e62703d71bf3302d.

What do you think?