lib: Fix some more compiler warnings

lib: Fix some more compiler warnings

Three different changes:
a) in nexthop.c we have an actual bug, we've rearranged
the enumerated types and the list of enums has changed, we
were displaying the wrong nexthop information.

b) Remove some dead code from sockunion.h

c) '\0' evaluates to 0 which is NULL.  vector_set takes
a pointer.

Signed-off-by: Donald Sharp <sharpd@cumulusnetworks.com>
Reviewed-by: Daniel Walton <dwalton@cumulusnetworks.com>