Wild address read in vorbis_decode_packet_rest 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 read in `DECODE` macro when `var` is negative. As it can be seen in the definition of `DECODE_RAW` a negative `var` is a valid value. This issue may be used to leak internal memory allocation information.