libtizplatform: allow null handles passed to tiz_event_io_destroy and tiz_event_timer_destroy

[tizonia]

Currently these two functions assert the handle against NULL

assert (NULL != ap_ev_io);
assert (NULL != ap_ev_timer);

which creates an obligation on client code to check the handles agains NULL before using these functions. This is not optimal. Better to replace the assert with an if statement so that the check is done in the library instead of in the client code.