Add some more casts in assert() statemens

Add some more casts in assert() statemens

This fixes the following warning messages of gcc 4.5.3 on Cygwin when
building with debug code enabled:

 ng_ipaddr.c: In function ‘ng_ipaddr_init’:
 ng_ipaddr.c:52:2: warning: comparison between signed and
                   unsigned integer expressions
 ng_ipaddr.c:53:20: warning: comparison between signed and
                    unsigned integer expressions

 resolve.c: In function ‘ForwardLookup’:
 resolve.c:271:3: warning: comparison between signed and
                  unsigned integer expressions