Fix: extra '%' causes babeltrace2 to abort() in logging code

Fix: extra '%' causes babeltrace2 to abort() in logging code

An extra '%' in a TRACE logging statement causes
bt_common_custom_vsnprintf() to interpret '!' as a conversion
specifier. This causes handle_conversion_specifier_bt() to invoke
abort().

Signed-off-by: Jérémie Galarneau <jeremie.galarneau@efficios.com>
Change-Id: Ia077616ea84559dfc3abb57d6404d06a6e9b7baf
Reviewed-on: https://review.lttng.org/c/babeltrace/+/1836
Tested-by: jenkins <jenkins@lttng.org>