Null pointer dereference in vorbis_deinit in stb_vorbis

stb_vorbis is a single file MIT licensed library for processing ogg vorbis files. A crafted file may trigger memory allocation failure in `start_decoder`. In that case the function returns early, the `f->comment_list` is set to `NULL`, but `f->comment_list_length` is not reset. Later in `vorbis_deinit` it tries to dereference the `NULL` pointer. This issue may lead to denial of service.