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

[chaoren]

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.

[clayborg]

lgtm

lldb/trunk/source/Host/posix/ConnectionFileDescriptorPosix.cpp

if (!got_lock)
         if (log)
             log->Printf("%p ConnectionFileDescriptor::Read () failed to get the connection lock.", static_cast<void *>(this));
         if (error_ptr)
             error_ptr->SetErrorString("failed to get the connection lock for read.");
 
         status = eConnectionStatusTimedOut;
         return 0;
     }
-    else if (m_shutting_down)
-        return eConnectionStatusError;
+
+    if (m_shutting_down)
+    {
+        status = eConnectionStatusError;
+        return 0;
+    }
 
     status = BytesAvailable(timeout_usec, error_ptr);
     if (status != eConnectionStatusSuccess)
         return 0;
 
     Error error;
     size_t bytes_read = dst_len;
     error = m_read_sp->Read(dst, bytes_read);