Assert when adding range that encloses an existing range

Adding 10.80.116.0/25 followed by 10.0.0.0/8 fails with assert(bits_in_common <= prefix_len).

I've not managed to build the node level code yet. I'm just playing with the btrie.cc level code, so it's possible I'm doing something wrong but I thought I'd mention it.

Some code that triggers the assert: https://gist.github.com/4107242
