Fix bug in bulk extending temp relation after failure

Fix bug in bulk extending temp relation after failure

A ResourceOwnerEnlarge() call was missing. That led to an error:

ERROR:  ResourceOwnerRemember called but array was full

and an assertion failure, if you tried to extend a temp relation again
after a failure. Alexander's test case used running out of disk space
to trigger the original failure.

This bug was introduced in the large ResourceOwner rewrite commit
b8bff07daa. Before that, the UnpinLocalBuffer() call guaranteed that
the subsequent PinLocalBuffer() will succeed, but after the rewrite,
releasing an old resource doesn't guarantee that there is space for a
new one.

Add a comment explaining why the UnpinBuffer + PinBuffer calls in
BufferAlloc(), with no ResourceOwnerEnlarge() in between, are safe.

Reported-by: Alexander Lakhin
Discussion: https://www.postgresql.org/message-id/dc574fea-c83e-a600-08cd-10881762e4fa@gmail.com