ash: fix corruption of ${#var} if $var contains UTF-8 characters

As reported in bug 8506:

   $ X=abcdÉfghÍjklmnÓpqrstÚvwcyz
   $ echo ${#X}
   abcd26

The result should be 26.

This regression was introduced by:

   <d68d1fb> 2015-05-18 [Ron Yorston]  ash: code shrink around varvalue

The length in characters was being used to discard the contents of
the variable instead of the length in bytes.

URL: https://bugs.busybox.net/8506
Reported-by: Martijn Dekker <martijn@inlv.org>
Signed-off-by: Ron Yorston <rmy@pobox.com>
Signed-off-by: Mike Frysinger <vapier@gentoo.org>

shell/ash_test/ash-vars/var-utf8-length.right

@@ -0,0 +1 @@
+26

shell/ash_test/ash-vars/var-utf8-length.tests

@@ -0,0 +1,2 @@
+X=abcdÉfghÍjklmnÓpqrstÚvwcyz
+echo ${#X}