Before subtracting a value from a prefix length, make sure the prefix

Before subtracting a value from a prefix length, make sure the prefix
length is >= the value.  (In theory, if that check isn't done, the
prefix length will overflow, and, as it's unsigned, that'd make it very
large, and rejected by the other sanity check - but it's cleaner to
check explicitly.)

In decode_rt_routing_info(), fix the checks for the prefix length -
according to draft-marques-ppvpn-rt-constrain-01, the default route
target has a 0-length prefix, and all others have a length between 32
and 96.