Fix: relayd: Dereference after null check

There is no legitimate case where a stream's trace chunk would be NULL
while receiving a data packet. It could only result from an internal
error. Hence, stream->trace_chunk != NULL can be considered a
pre-condition of this function.

Coverity report:
  CID 1404937 (#1 of 1): Dereference after null check (FORWARD_NULL)
  11. var_deref_model: Passing null pointer stream->index_file to
  relay_index_set_file, which dereferences it

Reported-by: Coverity (1404937) Dereference after null check
Signed-off-by: Francis Deslauriers <francis.deslauriers@efficios.com>
Signed-off-by: Jérémie Galarneau <jeremie.galarneau@efficios.com>