Multi-byte write heap buffer overflow in start_decoder in stb_vorbis

stb_vorbis is a single file MIT licensed library for processing ogg vorbis files. A crafted file may trigger out of bounds write in `f->vendor[i] = get8_packet(f);`. The root cause is an integer overflow in `setup_malloc`. A sufficiently large value in the variable `sz` overflows with `sz+7` in and the negative value passes the maximum available memory buffer check. This issue may lead to code execution.