Out of bounds heap buffer write in stb_vorbis

stb_vorbis is a single file MIT licensed library for processing ogg vorbis files. A crafted file may trigger memory write past an allocated heap buffer in `start_decoder`. The root cause is a potential integer overflow in `sizeof(char*) * (f->comment_list_length)` which may make `setup_malloc` allocate less memory than required. Since there is another integer overflow an attacker may overflow it too to force `setup_malloc` to return 0 and make the exploit more reliable. This issue may lead to code execution.