Fix LyX server on Windows

Fix LyX server on Windows

On some recent Windows versions, GetLastError() may also return
NO_ERROR instead of ERROR_IO_PENDING during an overlapped write
operation to a pipe. This was confusing the state machine in
Server.cpp so that replies to commands were scheduled but were
never actually output.

(cherry picked from commit cf5f2661dc0a902e541704172ab369ba3e5a54d6)