Fix: sessiond: snapshot errors don't clear session's trace chunk

The snapshot record command is implemented by creating and setting
a new trace chunk on the target session, capturing the snapshot, and
closing the session's trace chunk once it is complete.

On some error paths, the session's newly created trace chunk is not
cleared. This means that the session is seen to have a
'current_trace_chunk' on the next attempt to record a snapshot; an
unexpected condition for which an assert() exists.

This results in the following crash:
lttng-sessiond: /home/smarchi/src/lttng-tools/src/bin/lttng-sessiond/cmd.c:4685: snapshot_record: Assertion `!session->current_trace_chunk' failed.

Ensure that the session's current trace chunk is closed and cleared
when an error occurs during the command's execution.

Signed-off-by: Jérémie Galarneau <jeremie.galarneau@efficios.com>
Change-Id: I4f78e3c324bee873349ed5b4d8e189a000458a07