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.