isc_event_allocate() cannot fail, remove the fail handling blocks

isc_event_allocate() cannot fail, remove the fail handling blocks

isc_event_allocate() calls isc_mem_get() to allocate the event structure.  As
isc_mem_get() cannot fail softly (e.g. it never returns NULL), the
isc_event_allocate() cannot return NULL, hence we remove the (ret == NULL)
handling blocks using the semantic patch from the previous commit.