#238 Run time errors when built with --enable-undefined-sanitize

[Errol van de l'Isle]

Various run time errors show up when gpsim is built to detect undefined behaviour.
While running the regressions these runtime errors show up

ioports.h:286:29: runtime error: reference binding to null pointer of type 'struct IOPIN'

pic-ioports.cc:154:20: runtime error: member call on address 0x55d9461b01e0 which does not point to an object of type 'PicPortRegister'

pic-ioports.cc:132:10: runtime error: member access within address 0x55d84ab3e060 which does not point to an object of type 'PicPortRegister'

pic-ioports.cc:138:64: runtime error: member access within address 0x55d84ab3e060 which does not point to an object of type 'PicPortRegister'

a2dconverter.cc:1332:73: runtime error: downcast of address 0x560ffbbd4140 which does not point to an object of type 'IO_bi_directional_pu'

comparator.cc:149:20: runtime error: member call on address 0x55b27a6389e0 which does not point to an object of type 'CMCON'