Add test case.

This tests a codepath in QR_next_tuple that had no coverage before.
Add test case for large objects.
Fix Windows compilation with libpq enabled, but without SSPI

GetUserNameEx function requires secur32.dll. It is now referenced in the
code, regardless of SSPI, so we need to link with secur32.dll.
Fix a crash bug on closed connection reported by Jade Koskela.