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.
The differences between two files are very minimal and most of the
code is common. Merge those two files and use #ifdef WIN32 to include
the right bits on Windows.