agent: Fix use of freed stream in nice_agent_dispose()

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.
2 jobs for fix-agent-dispose in 2 minutes and 4 seconds (queued for 1 second)
Status Job ID Name Coverage
  Build
passed #68758
build autotools

00:01:37

passed #68759
build meson

00:02:03