Fix: lib: BT_ASSERT_PRE_NON_NULL_FROM_FUNC(): change `: ` -> `.` in msg.

Fix: lib: BT_ASSERT_PRE_NON_NULL_FROM_FUNC(): change `: ` -> `.` in msg.

This changes, for example,

    ...
    Error is:
    Value object is NULL:
    Aborting...

to

    ...
    Error is:
    Value object is NULL.
    Aborting...

Signed-off-by: Philippe Proulx <eeppeliteloop@gmail.com>
Change-Id: Ic122d50f5cb88dd72978d5dd8e2e4b22ee965a3e
Reviewed-on: https://review.lttng.org/c/babeltrace/+/3451
Reviewed-by: Simon Marchi <simon.marchi@efficios.com>
Tested-by: jenkins <jenkins@lttng.org>