minor compiler warning fixes and coding style

src/core/ipv4/autoip.c

@@ -456,6 +456,10 @@ autoip_tmr(void)
             }
           }
           break;
+
+        default:
+          /* nothing to do in other states */
+          break;
       }
     }
     /* proceed to next network interface */

src/core/ipv4/ip_frag.c

@@ -769,7 +769,7 @@ ip4_frag(struct pbuf *p, struct netif *netif, const ip4_addr_t *dest)
     }
     /* fill in the IP header */
     SMEMCPY(rambuf->payload, original_iphdr, IP_HLEN);
-    iphdr = rambuf->payload;
+    iphdr = (struct ip_hdr*)rambuf->payload;
 #else /* LWIP_NETIF_TX_SINGLE_PBUF */
     /* When not using a static buffer, create a chain of pbufs.
      * The first will be a PBUF_RAM holding the link and IP header.