Add some assertions to please coverity.

In event_process_active_single_queue, EVLIST_INIT must be set on any
event that uses one of the event-only closures, and so "ev" will be
set in those cases.  But coverity's worried here (CIDs numerous).  So
instead, just add the assertions that should make it happy.
Move event_debug_note_teardown_ before mm_free.

This isn't a bug, since only the pointer value of ev was used, but
it's probably best not to tempt fate.  Found by coverity.
Fix an unlikely but possible error case for http connections

Found by coverity
sample/le-proxy: Fail more gracefully if opening listener fails
Use void casts to suppress some "unchecked return value" warns
Fix a (failure-only) null dereference in the unit tests
Fix a logic error in test_evbuffer_freeze
Add missing check to test_evbuffer_file_segment_add_cleanup_cb
Fix some crash-on-fail cases in DNS regression tests

Found by coverity.
DNS tests: add a missing check
Finalize tests: add a missing check
test_evutil_rtrim: add another missing check.
Add an assertion for another of the complaints from coverity. See 1b065d07df196