Off-by-one heap buffer 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 buffer write in `start_decoder` because at maximum `m->submaps` can be 16 but `submap_floor` and `submap_residue` are declared as arrays of 15 elements. This issue may lead to code execution.