ESPAsyncWebServer multipart parser has null-pointer write

ESPAsyncWebServer is an asynchronous HTTP and WebSocket server library for ESP32, ESP8266, RP2040 and RP2350. Prior to version 3.11.2, the multipart parser in src/WebRequest.cpp frees _itemBuffer and sets it to NULL when BOUNDARY_OR_DATA matches the closing boundary of a file part, before confirming the following terminator, while leaving _itemIsFile true. If the next byte is neither the final closing dashes nor a clean CRLF, the DASH3_OR_RETURN2 or EXPECT_FEED2 rewind branch re-injects provisionally consumed bytes through itemWriteByte. Because _itemIsFile remains true, _itemBuffer[_itemBufferIndex++] writes through a null pointer. On ESP32 this causes a StoreProhibited exception, panic and device reboot, producing remote denial of service. A route with a non-trivial body/upload handler, such as a file-upload or web-OTA endpoint, must be present. This issue is fixed in version 3.11.2.