assrender: Make sure that the video buffer is writable before writing to it

ext/assrender/gstassrender.c

@@ -1024,6 +1024,7 @@ gst_ass_render_chain_video (GstPad * pad, GstBuffer * buffer)
       return ret;
     }
 
+    buffer = gst_buffer_make_writable (buffer);
     render->blit (render, ass_image, buffer);
   }