bufmgr: Fix undefined behaviour with, unrealistically, large temp_buffers

bufmgr: Fix undefined behaviour with, unrealistically, large temp_buffers

Quoting Melanie:
> Since if buffer is INT_MAX, then the -(buffer + 1) version invokes
> undefined behavior while the -buffer - 1 version doesn't.

All other places were already using the correct version. I (Andres), copied
the code into more places in a patch. Melanie caught it in review, but to
prevent more people from copying the bad code, fix it. Even if it is a
theoretical issue.

We really ought to wrap these accesses in a helper function...

As this is a theoretical issue, don't backpatch.

Reported-by: Melanie Plageman <melanieplageman@gmail.com>
Discussion: https://postgr.es/m/CAAKRu_aW2SX_LWtwHgfnqYpBrunMLfE9PD6-ioPpkh92XH0qpg@mail.gmail.com