Recent kernels don't include RTA_PRIORITY when it's 0.

Recent kernels don't include RTA_DST in the default route.  This fixes
incorrect parsing of such entries.  Reported by Robert Lukan.

It was difficult to write, there's no reason it should easy to read.

This saves 512 bytes of BSS.

This is safe, since no part of the code ever holds a pointer to an xroute.