cpoll: check for EventHandlerData on incoming event;
this fixes a segfault in cppsp that happens as a result of a callback getting called twice because a hang-up event occurred on a file descriptor;
hang-up events are internally translated to Events::all (and with EventData::hungUp set to true), in order to trigger all un-called callbacks.
before this commit, stale callback functions would get called when a hang-up occurs.
cpoll: StringPool: added memory diagnostic capabilities (sets newly allocated memory to 0xFF and freed memory to 0xFE);
enabled only if STRINGPOOL_MEMORY_DIAG is defined