sandbox: Fix warnings in cpu.c and os.c

This fixes the following two problems:

cppcheck reports:
[arch/sandbox/cpu/start.c:132]: (error) Uninitialized variable: err
[arch/sandbox/cpu/os.c:371]: (error) Memory leak: fname

Signed-off-by: Simon Glass <sjg@chromium.org>
Reported-by: Wolfgang Denk <wd@denx.de>