[snapshot] Save if IOStream has |m_buf| set
IOStream uses |m_buf| as a buffer pointer and as a flag at the
same time: if it's not set then there's no need to flush.
Previous snapshotting code didn't save this bit of information,
so next time one tried to allocate a buffer from a just loaded
empty IOStream, it thought it's not empty (as |m_buf| was not
nullptr) and tried to flush garbage.
This CL saves this separately, so now |m_buf| after snapshot
loading is exactly in the same state as it was before.
Bug=b/36700704
Change-Id: I7af309bb1d3c16ba20969f7bb3f9bfc067d0051d
1 file changed
