sprintf bug (concerning %c for 8-bit character in non-UTF8 mode) fixed

char *str points to SLuchar_Type utf8_buf[], which had too small scope

src/test/strops.sl failed in the following environment:
 - compiler:  gcc (Debian 4.7.2-5) 4.7.2
 - CFLAGS:    -g -O2
 - libc:      GNU C Library (Debian EGLIBC 2.13-38) stable release version 2.13
 - kernel:    3.2.0-4-686-pae #1 SMP Debian 3.2.39-2 i686 GNU/Linux