Fully check the second strtoul() call in v2i_IPAddrBlocks()

Fully check the second strtoul() call in v2i_IPAddrBlocks()

This can read a value in an arbitrary base from a string that is
supposed to be followed by whitespace or a colon, so it cannot be
switched to strtonum(). The current checks don't allow a read past
the end, but let's use the standard idiom instead.

ok jsing