bgpd, lib, nhrpd, zebra: verify return of sockunion2hostprefix

bgpd, lib, nhrpd, zebra: verify return of sockunion2hostprefix

The return from sockunion2hostprefix tells us if the conversion
succeeded or not.  There are places in the code where we
always assume that it just `works`, since it can fail
notice and try to do the right thing.

Please note that failure of this function for most cases
of sockunion2hostprefix is highly highly unlikely as that
the sockunion was already created and tested elsewhere
it's just that this function can fail.

Signed-off-by: Donald Sharp <sharpd@nvidia.com>