fix warnings: expression which evaluates to zero treated as null pointer

clang 3.4 brings some fresh air

src/libknot/dnssec/sign.c

@@ -1082,7 +1082,7 @@ static int init_algorithm_data(const knot_key_params_t *params,
 	assert(params);
 	assert(data);
 
-	knot_dnssec_key_data_t result = { '\0' };
+	knot_dnssec_key_data_t result = { 0 };
 
 	result.functions = get_implementation(params->algorithm);
 	if (!result.functions) {

src/utils/common/netio.c

@@ -154,7 +154,7 @@ static void get_addr_str(const struct sockaddr_storage *ss,
 	if (*dst != NULL) {
 		int ret = snprintf(*dst, buflen, "%s(%s)", addr_str, sock_name);
 		if (ret <= 0 || ret >= buflen) {
-			*dst = '\0';
+			**dst = '\0';
 		}
 	}
 }