Fix: event notification capture error handling

Fix: event notification capture error handling

When the captured fields end up taking more than

  PIPE_BUF - sizeof(struct lttng_ust_abi_event_notifier_notification) - 1

bytes of space for the msgpack message, the notification append capture
fails.

Currently, the result is that the msgpack buffer will contain a (likely
corrupted) truncated msgpack data.

Handle those overflow errors, and when they are encountered, reset the
msgpack writer position to skip the problematic captured field entirely.

Change-Id: I7ba1bf06aa72512fc73211a1d8ae6823d0e8d7ff