lib: clear error in clock_snapshots_are_monotonic_one

lib: clear error in clock_snapshots_are_monotonic_one

When bt_clock_snapshot_get_ns_from_origin is called in
clock_snapshots_are_monotonic_one, it can return OVERFLOW_ERROR, in
which case it also sets the current thread error.
clock_snapshots_are_monotonic_one doesn't report an error status to its
caller (it's not its goal), so it should not leave the error there.

This is exercised when running Python test
ClockSnapshotTestCase.test_clock_class.

Change-Id: I0f15c2607c299fabc3a89e6a292d97d8d5815fc4
Signed-off-by: Simon Marchi <simon.marchi@efficios.com>
Reviewed-on: https://review.lttng.org/c/babeltrace/+/2360
Tested-by: jenkins <jenkins@lttng.org>
Reviewed-by: Philippe Proulx <eeppeliteloop@gmail.com>