lib: internal: add BT_ASSERT_POST()

lib: internal: add BT_ASSERT_POST()

BT_ASSERT_POST() is just like BT_ASSERT_PRE(), but it is used to
validate the returned values of user functions and methods.

The only difference with BT_ASSERT_PRE() is that its error message
indicates that a postcondition was not satisfied instead of a
precondition. This is simply more accurate.

Signed-off-by: Philippe Proulx <eeppeliteloop@gmail.com>
Change-Id: I91815301ada19b42ba58f32db7b9c412cbe641e0
Reviewed-on: https://review.lttng.org/c/babeltrace/+/1574
Tested-by: jenkins <jenkins@lttng.org>
Reviewed-by: Simon Marchi <simon.marchi@efficios.com>