Heap buffer out of bounds write 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[len] = (char)'\0';`. The root cause is that if `len` read in `start_decoder` is a negative number and `setup_malloc` successfully allocates memory in that case, but memory write is done with a negative index `len`. Similarly if len is INT_MAX the integer overflow len+1 happens in `f->vendor = (char*)setup_malloc(f, sizeof(char) * (len+1));` and `f->comment_list[i] = (char*)setup_malloc(f, sizeof(char) * (len+1));`. This issue may lead to code execution.