trace{-class}.c: BT_ASSERT_PRE() -> BT_ASSERT_POST() (ref. count check)

trace{-class}.c: BT_ASSERT_PRE() -> BT_ASSERT_POST() (ref. count check)

Those are postcondition assertions because they check some state after a
user function returns.

Signed-off-by: Philippe Proulx <eeppeliteloop@gmail.com>
Change-Id: I3705ee451ff8ae024a2eb17fbe6ae9cc2b11d05e
Reviewed-on: https://review.lttng.org/c/babeltrace/+/2127
Tested-by: jenkins <jenkins@lttng.org>
Reviewed-by: Francis Deslauriers <francis.deslauriers@efficios.com>