Fix an illegal read error in the evbuffer_add_reference tests

Found with AddressSanitizer
Fix a use-after-free error on EV_CLOSURE_EVENT_FINALIZE callbacks

After running the callback, we were checking evcb->evcb_closure to
decide whether to call mm_free(ev).  But the callback itself might
have freed ev, so we need to grab that field first

Found with AddressSanitizer
Fix ubsan warnings when parsing ipv4/ipv6 addrs

left-shifting a one-byte integer by 24 invokes undefined behavior.
Let's not do that.
Catch over-large port numbers early in http

Otherwise integer overflow potentially turns the port number into garbage.
evtag: detect tags over 32-bits earlier
Remove integer-overflow unit tests

There's not much point checking undefined behavior.