nice_stream_close() calls agent_find_stream(), which iterates over
agent->streams list.

Therefore, when we're freeing agent->streams in a loop, we must also
delete the respective GSList item from the list at the end of each
iteration. Otherwise agent_find_stream() would stumble over already
deleted NiceStream instance the next time it gets called.

Fixes random crashes when running test-different-number-streams on
Windows.