bug#5526: 23.1; (master) - charset.c possible unwise use of alloca(3)

[Vivek Dasmohapatra]

> Actually, after looking at this further, I think we need to use
> SAFE_ALLOCA instead of xmalloc.  I've changed the code accordingly.

Whatever you think is best... I wasn't sure if SAFE_ALLOCA was the right 
thing to use, since the size of the struct was so much > than MAX_ALLOCA
that the xmalloc path would always be taken.

I suspect it's unlikely to impact performance as I don't imagine emacs 
spends a lot of time loading charsets over and over again.