Fix: consumerd: NULL pointer dereference during metadata sync

Fix: consumerd: NULL pointer dereference during metadata sync

The following crash was reported when short-lived applications
are traced in a live session with per-pid buffering channels.

From the original report:

```
 Thread 1 (Thread 0x7f72b67fc700 (LWP 1912155)):
 #0  0x00005650b3f6ccbd in commit_one_metadata_packet (stream=0x7f729c010bf0) at ust-consumer.c:2537
 #1  0x00005650b3f6cf58 in lttng_ustconsumer_sync_metadata (ctx=0x5650b588ce60, metadata=0x7f729c010bf0) at ust-consumer.c:2608
 #2  0x00005650b3f4dba3 in do_sync_metadata (metadata=0x7f729c010bf0, ctx=0x5650b588ce60) at consumer-stream.c:471
 #3  0x00005650b3f4dd3c in consumer_stream_sync_metadata (ctx=0x5650b588ce60, session_id=0) at consumer-stream.c:548
 #4  0x00005650b3f6de78 in lttng_ustconsumer_read_subbuffer (stream=0x7f729c0058e0, ctx=0x5650b588ce60) at ust-consumer.c:2917
 #5  0x00005650b3f45196 in lttng_consumer_read_subbuffer (stream=0x7f729c0058e0, ctx=0x5650b588ce60) at consumer.c:3524
 #6  0x00005650b3f42da7 in consumer_thread_data_poll (data=0x5650b588ce60) at consumer.c:2894
 #7  0x00007f72bdc476db in start_thread (arg=0x7f72b67fc700) at pthread_create.c:463
 #8  0x00007f72bd97088f in clone () at ../sysdeps/unix/sysv/linux/x86_64/clone.S:95

The segfault happen on the access to 'stream->chan->metadata_cache->lock'
chan value here is zero.
```

The problem is easily reproducible if a sleep(1) is added just after
the call to lttng_ustconsumer_request_metadata(), before the metadata
stream lock is re-acquired.

During the execution of the "request_metadata", an application can
close. This will cause the session daemon to push any remaining
metadata to the consumer daemon and to close the metadata channel.

Closing the metadata channel closes the metadata stream's wait_fd,
which is an internal pipe. The closure of the metadata pipe is
detected by the metadata_poll thread, which will ensure that all
metadata has been consumed before issuing the deletion of the metadata
stream and channel.

During the deletion, the channel's "stream" attribute the stream's
"chan" attribute are set to NULL as both are logically deleted and
should not longer be used.

Meanwhile, the thread executing commit_one_metadata_packet()
re-acquires the metadata stream lock and trips on the now-NULL "chan"
member.

The fix consists in checking if the metadata stream is logically
deleted after its lock is re-acquired. It is correct for the
sync_metadata operation to then complete successfully as the metadata
is synced: the metadata guarantees this before deleting the
stream/channel.

Since the metadata stream's lifetime is protected by its lock, there
may be other sites that need such a check. The lock and deletion check
could be combined into a single consumer_stream_lock() helper in
follow-up fixes.

Reported-by: Jonathan Rajotte <jonathan.rajotte-julien@efficios.com>
Signed-off-by: Jérémie Galarneau <jeremie.galarneau@efficios.com>