muxread: remove unreachable code
muxread: remove unreachable code

IDX_UNKNOWN chunks are handled separately in WebPMuxGetChunk()

Change-Id: Iaa0d3a1cd1080264c27671637551f52f71d73b07

PKRoma/libwebp,PKRoma/libwebp,PKRoma/libwebp,webmproject/libwebp,PKRoma/libwebp,PKRoma/libwebp,PKRoma/libwebp,webmproject/libwebp,webmproject/libwebp,webmproject/libwebp,webmproject/libwebp