lib: If two istreams share one parent, i_stream_get_data() may have returned corrupte...

lib: If two istreams share one parent, i_stream_get_data() may have returned corrupted data to another.
This happened only for istreams that used parent's buffer directly instead
of having their own buffer. For now at least we've solved this by truncating
the other stream's buffer so it needs to be read again. Hopefully this is
good enough.

Added also unit test to check this functionality.