Fixed LWIP warning issues found by Coverity scan

Author: tymoteuszblochmobica

features/lwipstack/lwip/src/api/lwip_api_msg.c

@@ -1332,7 +1332,7 @@ lwip_netconn_do_connected(void *arg, struct tcp_pcb *pcb, err_t err)
   conn->state = NETCONN_NONE;
   API_EVENT(conn, NETCONN_EVT_SENDPLUS, 0);
 
-  if (was_blocking) {
+  if (was_blocking && op_completed_sem != NULL) {
     sys_sem_signal(op_completed_sem);
   }
   return ERR_OK;

features/lwipstack/lwip/src/core/ipv4/lwip_ip4_frag.c

@@ -206,7 +206,9 @@ ip_reass_free_complete_datagram(struct ip_reassdata *ipr, struct ip_reassdata *p
     pbuf_free(pcur);
   }
   /* Then, unchain the struct ip_reassdata from the list and free it. */
-  ip_reass_dequeue_datagram(ipr, prev);
+  if (prev != NULL) {
+    ip_reass_dequeue_datagram(ipr, prev);
+  }
   LWIP_ASSERT("ip_reass_pbufcount >= pbufs_freed", ip_reass_pbufcount >= pbufs_freed);
   ip_reass_pbufcount = (u16_t)(ip_reass_pbufcount - pbufs_freed);
 
@@ -660,8 +662,9 @@ ip4_reass(struct pbuf *p)
     }
 
     /* release the sources allocate for the fragment queue entry */
-    ip_reass_dequeue_datagram(ipr, ipr_prev);
-
+    if (ipr_prev != NULL) {
+      ip_reass_dequeue_datagram(ipr, ipr_prev);
+    }
     /* and adjust the number of pbufs currently queued for reassembly. */
     clen = pbuf_clen(p);
     LWIP_ASSERT("ip_reass_pbufcount >= clen", ip_reass_pbufcount >= clen);

features/lwipstack/lwip/src/core/lwip_dns.c

@@ -431,7 +431,7 @@ dns_add_interface_server(u8_t numdns, const char *interface_name, const ip_addr_
   }
   // add new dns server to the list tail
   new_interface_server = mem_malloc(sizeof(struct dns_server_interface));
-  strncpy(new_interface_server->interface_name, interface_name, INTERFACE_NAME_MAX_SIZE);
+  snprintf(new_interface_server->interface_name, INTERFACE_NAME_MAX_SIZE, "%s",interface_name);
   new_interface_server->dns_servers[numdns] = (*dnsserver);
   new_interface_server->next = NULL;