lib: add precond. check for begin <= end on pkt./ev. disc. msg. creation

lib: add precond. check for begin <= end on pkt./ev. disc. msg.  creation

Add a precondition check in:

- bt_message_discarded_events_create_with_default_clock_snapshots
- bt_message_discarded_packets_create_with_default_clock_snapshots

Verify that the beginning clock snapshot value is less-than or equal-to
the end clock snapshot.

A failure looks like this:

    04-12 00:01:37.519 653088 653088 F LIB/MSG-DISCARDED-ITEMS bt_message_discarded_events_create_with_default_clock_snapshots@discarded-items.c:221 Babeltrace 2 library precondition not satisfied; error is:
    04-12 00:01:37.519 653088 653088 F LIB/MSG-DISCARDED-ITEMS bt_message_discarded_events_create_with_default_clock_snapshots@discarded-items.c:221 Beginning default clock snapshot value is greater than end default clock snapshot value: cs-begin-val=20, cs-end-val=10, msg-iter-addr=0x611000001bc0, msg-iter-upstream-comp-name="ze source", msg-iter-upstream-comp-log-level=NONE, msg-iter-upstream-comp-class-type=SOURCE, msg-iter-upstream-comp-class-name="MySrc", msg-iter-upstream-comp-class-partial-descr=""
    04-12 00:01:37.519 653088 653088 F LIB/MSG-DISCARDED-ITEMS bt_message_discarded_events_create_with_default_clock_snapshots@discarded-items.c:221 Aborting...

Change-Id: I1d4cc820d9daeff0fd3a80262c36609fe10a1958
Signed-off-by: Simon Marchi <simon.marchi@efficios.com>
Reviewed-on: https://review.lttng.org/c/babeltrace/+/3389
Tested-by: jenkins <jenkins@lttng.org>
Reviewed-by: Philippe Proulx <eeppeliteloop@gmail.com>