ClientContext:_write_some: release buffer once data really accepted for sending

may fix esp8266#1872

Author: d-a-v

libraries/ESP8266WiFi/src/include/ClientContext.h

@@ -447,8 +447,8 @@ class ClientContext
             }
             err_t err = tcp_write(_pcb, buf, next_chunk, TCP_WRITE_FLAG_COPY);
             DEBUGV(":wrc %d %d %d\r\n", next_chunk, will_send, (int) err);
-            _datasource->release_buffer(buf, next_chunk);
             if (err == ERR_OK) {
+                _datasource->release_buffer(buf, next_chunk);
                 _written += next_chunk;
                 need_output = true;
             } else {