Fix: leak of trace_path on error in ust_app_snapshot_record

trace_path is leaked in some error paths of ust_app_snapshot_record().
Lift trace_path to the function's scope and free it whenever it is not
NULL. This is not clean, but this function should be cleaned-up in a
separate patch.

Moreover, this fixes a use after free in the PER_PID-case as
trace_path is free'd when a channel is not found.

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