Fix use-after-free when astream fail to initialize.

When nn_astream_init fails, bstream was adding an invalid object to the
astreams list. This leads to "impossible" failures later.

This patch add the astream to the bstream->astreams list inside
nn_astream_init, before calling nn_stream_init. If nn_stream_init fails
when the astream is included in the astreams list, it is removed
properly.

This change fixes the random crashes in the tcp_shutdown test. The test
runs now 200 time in a loop without an error.

Signed-off-by: Martin Sustrik <sustrik@250bpm.com>