[IPv6addr] overrun in find_if() for 128bit prefixes
while reading over the IPv6addr code I notices that there is an overrun in
find_if() in the case where the prefix is 128. In this case,
mask.s6_addr[16] will be accessed twice, but that array only
has 16 elements.

The patch below takes the simple approach of just treating 128 as a corner
case and skiping the offending parts of the mask manipulation accordingly.
It also reverses the way the mask is seeded, removing bits rather than
adding them, to ensure that the corner case is all 1s rather than all 0s.

--HG--
extra : convert_revision : b4bc188b4ebe94824e042a674770c90ee4335469