traffic-selector: Use single buffer for both address families

traffic-selector: Use single buffer for both address families

The generic field of size 0 in the union that was used previously
triggered index-out-of-bounds errors with the UBSAN sanitizer that's
used on OSS-Fuzz.  Since the two family specific union members don't
really provide any advantage, we can just use a single buffer for both
families to avoid the errors.