Fix incorrect sentinel byte logic in GenerationRealloc()

Fix incorrect sentinel byte logic in GenerationRealloc()

This only affects MEMORY_CONTEXT_CHECKING builds.

This fixes an off-by-one issue in GenerationRealloc() where the
fast-path code which tries to reuse the existing allocation if the
existing chunk is >= the new requested size.  The code there thought it
was always ok to use the existing chunk, but when oldsize == size there
isn't enough space to store the sentinel byte.  If both sizes matched
exactly set_sentinel() would overwrite the first byte beyond the chunk
and then subsequent GenerationRealloc() calls could then fail the
Assert(chunk->requested_size < oldsize) check which is trying to ensure
the chunk is large enough to store the sentinel.

The same issue does not exist in aset.c as the sentinel checking code
only adds a sentinel byte if there's enough space in the chunk.

Reported-by: Alexander Lakhin <[email protected]>
Discussion: https://postgr.es/m/49275921-7b39-41af-5eb8-97b50ce3312e@gmail.com
Backpatch-through: 16, where the problem was introduced by 0e480385e