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

src/ipaddr/ng_ipaddr.c

@@ -49,8 +49,8 @@ ng_ipaddr_init(ng_ipaddr_t *addr, const char *ip_str, UINT16 port)
 	if (ret != 0)
 		return false;
 
-	assert(sizeof(*addr) >= res0->ai_addrlen);
-	if (sizeof(*addr) >= res0->ai_addrlen)
+	assert(sizeof(*addr) >= (size_t)res0->ai_addrlen);
+	if (sizeof(*addr) >= (size_t)res0->ai_addrlen)
 		memcpy(addr, res0->ai_addr, res0->ai_addrlen);
 	else
 		ret = -1;

src/ngircd/resolve.c

@@ -268,7 +268,7 @@ ForwardLookup(const char *hostname, array *IpAddr, int af)
 	}
 
 	for (a = ai_results; a != NULL; a = a->ai_next) {
-		assert(a->ai_addrlen <= sizeof(addr));
+		assert((size_t)a->ai_addrlen <= sizeof(addr));
 
 		if ((size_t)a->ai_addrlen > sizeof(addr))
 			continue;