Prevent reading from beyond the start of token data.
Prevent reading from beyond the start of token data.

svn path=/trunk/libcss/; revision=5807

zhuyadong/libcss,zhuyadong/libcss,zhuyadong/libcss,prepare/netsurf_libcss,servo/libcss,prepare/netsurf_libcss,prepare/netsurf_libcss,servo/libcss,servo/libcss