bt2: validate parameters to _StreamClass.create_event_class before creating the nativ...

For the same reasons outlined in d3bf1370a437 ("bt2: validate parameters
to _TraceClass.create_stream_class before creating the native object"),
validate the parameters when creating an event class before creating the
native object.

Change-Id: I435b21c145e663a373f3e989feac3e1d5de626c8
Signed-off-by: Simon Marchi <simon.marchi@efficios.com>
Reviewed-on: https://review.lttng.org/c/babeltrace/+/2415