Single-byte operations on vars broken

Hi!
It seems that this commit c5bf9702 changed behavior of ${var#?} under 
LC_ALL=C, which should consider characters to be one byte long and cut 
accordingly.

This code:

	var=联;
	LC_ALL=C
	while [ -n "$var" ]; do
		cut_var=${var#?}
		byte=${var%"$cut_var"}
		printf '>%02x<\n' "'$byte"
		var=$cut_var
	done
	unset LC_ALL

should output:

 >e8<
 >81<
 >94<

It works reliably in sh 0.5.12, also ash, bash, zsh (both in POSIX and 
native modes), but is broken in dash 0.5.13.

More context can be found at:
https://github.com/Vladimir-csp/app2unit/issues/17