Make IPAddressFamily_cmp() more pleasing on the eye

Make IPAddressFamily_cmp() more pleasing on the eye

Define and use MINIMUM() instead of a ternary operator and separate
the code from the declarations. Also, we can spare a line to make the
return legible instead of squeezing it into another ternary operator.

addressFamily->data contains a two-bytes AFI and an optional one-byte
SAFI.  This function currently also compares any trailing garbage that
may be present. Since comparison functions can't really error, this
needs to be checked bofore it is used. Such checks will be added in
subsequent commits.

ok jsing