Add thread safety and lifecycle for native pointers

[edde746]

Summary

• Add null guards to all JNI functions that dereference native pointers (prevents crash when pointer is 0)
• Add explicit release() methods to Ass, AssTrack, and AssRender (replaces GC-only finalize() cleanup)
• Add ReentrantLock in AssRender to synchronize renderFrame, setTrack, and release
• Synchronize AssHandler.readTrackDialogue() with the render lock to prevent concurrent ass_process_chunk / ass_render_frame on the same track
• Add AssHandler.release() for explicit native resource cleanup

Context

Google Play Console reports native crashes in libass.so (ass_parse_tags, ass_render_frame) caused by concurrent thread access to ASS_Track* and null/freed native pointers.

[moi15moi]

I believe to be properly thread safe, any call to libass should use the same mutex.