Fix: trace-chunk: dereference after NULL check

old_path is used directly even though it is checked for NULL. The
situation highlighted by Coverity does not appear to be possible given
the current use of the API. However, it should still be checked to
catch future errors (or current bugs).

1412200 Dereference after null check

Either the check against null is unnecessary, or there may be a null
pointer dereference.

In lttng_trace_chunk_rename_path_no_lock: Pointer is checked against
null but then dereferenced anyway (CWE-476)

Reported-by: Coverity Scan
Signed-off-by: Jérémie Galarneau <jeremie.galarneau@efficios.com>
Change-Id: I991231cc636eaed98cb84eec08a5072748ff9ef4