[update] avoid sscanf

It turnes out glibc's sscanf reads the whole string even though only a
small amount of it is necessary:

    https://sourceware.org/bugzilla/show_bug.cgi?id=17577

This was a particularly large performance hit when reading the table
file of a large grammar.

Signed-off-by: Shahbaz Youssefi <ShabbyX@gmail.com>