Fix bug where an additional O packet is sent after inferior exits.

Summary:
ConnectionFileDescriptor::Read was returning eConnectionStatusError instead of 0
on m_shutting_down, which caused the caller to think that some number of bytes
were read.

Reviewers: jingham, vharron, clayborg

Reviewed By: clayborg

Subscribers: lldb-commits

Differential Revision: http://reviews.llvm.org/D8850

llvm-svn: 234341