Corrected thread problem causing some calls to hang for 25s

Since the connection lock is released for a short while in
_dbus_connection_acquire_io_path there can already be a method return
received by another thread. The fix is to do an extra check after the
I/O path has been aquired both.

Bug: 2881601
Dr No: jsh / eastham

Change-Id: I7d3567d850cc2f8150db5ac68cffeb79a9fe0a72