* fns.c (concat): Minor tuning based on overflow analysis.

* fns.c (concat): Minor tuning based on overflow analysis.

This doesn't fix any bugs.  Use int to hold character, instead
of constantly refetching from Emacs object.  Use XFASTINT, not
XINT, for value known to be a character.  Don't bother comparing
a single byte to 0400, as it's always less.