Stub out 'close' call on m_master_fd for Windows.

PseudoTerminal.cpp uses a dummy implementation of posix_openpt for Windows. This
implementation just returns 0. So m_master_fd is 0. But destructor calls 'close'
on m_master_fd. This 'close' calls seems un-necessary as m_master_fd was never
opened in first place and calling 'close' on 0 can have other un-intended 
consequences.

I am committing it as obvious as it is only a one-liner. Long term, we may want
to refactor this class.

llvm-svn: 220705