If libpq ran out of memory while constructing the result set, it would hang,
waiting for more data from the server, which might never arrive. To fix,
distinguish between out-of-memory error and not-enough-data cases, and give
a proper error message back to the client on OOM.

There are still similar issues in handling COPY start messages, but let's
handle that as a separate patch.

Michael Paquier, Amit Kapila and me. Backpatch to all supported versions.