The plugin streams are a bit of a disaster area: there are 2 of them
with a common base class and the PluginInputStream is a UNO service.

The m_pPlugin gets reset in PluginStream::setMode(-1), which is called
by the plugin itself.  So those PluginStream/PluginInputStream methods
that are called via UNO (including dtors) need to check that m_pPlugin
isn't null, but they also lock member access with m_pPlugin's mutex.

Try to ensure that that works by ensuring that the plugin is still alive
with a WeakReference, *and* checking that m_pPlugin isn't null.

Change-Id: I925b30dd7cad3d3587fcc6b10f888e30d45fc38a