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/DataSource.h

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