Fix bogus use of "long" in AllocSetCheck()

Fix bogus use of "long" in AllocSetCheck()

Because long is 32-bit on 64-bit Windows, it isn't a good datatype to
store the difference between 2 pointers.  The under-sized type could
overflow and lead to scary warnings in MEMORY_CONTEXT_CHECKING builds,
such as:

WARNING:  problem in alloc set ExecutorState: bad single-chunk %p in block %p

However, the problem lies only in the code running the check, not from
an actual memory accounting bug.

Fix by using "Size" instead of "long".  This means using an unsigned
type rather than the previous signed type.  If the block's freeptr was
corrupted, we'd still catch that if the unsigned type wrapped.  Unsigned
allows us to avoid further needless complexities around comparing signed
and unsigned types.

Author: David Rowley <dgrowleyml@gmail.com>
Reviewed-by: Michael Paquier <michael@paquier.xyz>
Reviewed-by: Tom Lane <tgl@sss.pgh.pa.us>
Backpatch-through: 13
Discussion: https://postgr.es/m/CAApHDvo-RmiT4s33J=aC9C_-wPZjOXQ232V-EZFgKftSsNRi4w@mail.gmail.com