Fix: uninitialized variable

Fix: uninitialized variable

query.c: In function 'trace_info_query':
query.c:246:2: warning: 'stream_class' may be used uninitialized in this function [-Wmaybe-uninitialized]
  bt_put(stream_class);
  ^
query.c:216:30: note: 'stream_class' was declared here
  struct bt_ctf_stream_class *stream_class;
                              ^

Signed-off-by: Mathieu Desnoyers <mathieu.desnoyers@efficios.com>
Signed-off-by: Jérémie Galarneau <jeremie.galarneau@efficios.com>