add more C89-like targets; fix related stuff

src/net_builtin.c

@@ -972,7 +972,7 @@ static void rx_ndp_ra(struct mg_tcpip_if *ifp, struct pkt *pkt) {
   bool gotl2addr = false, gotprefix = false, changed = false;
   uint8_t l2[sizeof(struct mg_l2addr)];
   uint32_t mtu = 0;
-  uint8_t *prefix, prefix_len;
+  uint8_t *prefix = NULL, prefix_len = 0;
 
   if (pkt->pay.len < sizeof(*ra)) return;
   if (ifp->state6 == MG_TCPIP_STATE_UP) {