lib: Check prefix length from zebra is sensible

lib: Check prefix length from zebra is sensible

* zclient.c: prefix length on router-id and interface address add
  messages not sanity checked.  fix.

* */*_zebra.c: Prefix length on zebra route read was not checked, and
  clients use it to write to storage.  An evil zebra could overflow
  client structures by sending overly long prefixlen.

Prompted by discussions with:

Donald Sharp <sharpd@cumulusnetworks.com>