Handle LTTNG_EVENT_FIELD_VALUE_STATUS_UNAVAILABLE in capture example

[compudj]

In handle_evaluation()

we have: assert(event_field_value_status == LTTNG_EVENT_FIELD_VALUE_STATUS_OK);

with recent changes in lttng-modules, when a page fault occurs, the resulting field is a event_field_value_status == LTTNG_EVENT_FIELD_VALUE_STATUS_UNAVAILABLE. If would be good if the example handles this case by skipping the capture rather than asserting.