Fix warnings (#2881)

* Suppressed -Wunused-parameter and -Wunused-function by casting to void unused identifiers.

* Explicit initialization of all fields to suppress -Wmissing-field-initializers.

* Fixed signed/unsigned integer comparison.

* memset initialization of structs.

* More -Wunused-parameter fixes.

libraries/ESP8266WiFi/src/include/ClientContext.h

@@ -372,6 +372,8 @@ protected:
 
     err_t _sent(tcp_pcb* pcb, uint16_t len)
     {
+        (void) pcb;
+        (void) len;
         DEBUGV(":sent %d\r\n", len);
         _write_some_from_cb();
         return ERR_OK;
@@ -405,6 +407,8 @@ protected:
 
     recv_ret_t _recv(tcp_pcb* pcb, pbuf* pb, err_t err)
     {
+        (void) pcb;
+        (void) err;
         if(pb == 0) { // connection closed
             DEBUGV(":rcl\r\n");
             _cancel_write();
@@ -425,6 +429,7 @@ protected:
 
     void _error(err_t err)
     {
+        (void) err;
         DEBUGV(":er %d %08x\r\n", err, (uint32_t) _datasource);
         tcp_arg(_pcb, NULL);
         tcp_sent(_pcb, NULL);

libraries/ESP8266WiFi/src/include/DataSource.h

@@ -75,6 +75,7 @@ public:
 
     void release_buffer(const uint8_t* buffer, size_t size) override
     {
+        (void) buffer;
         _pos += size;
     }
 

libraries/ESP8266WiFi/src/include/UdpContext.h

@@ -330,6 +330,9 @@ private:
     void _recv(udp_pcb *upcb, pbuf *pb,
             ip_addr_t *addr, u16_t port)
     {
+        (void) upcb;
+        (void) addr;
+        (void) port;
         if (_rx_buf)
         {
             // there is some unread data