sandbox: Add os_realloc()

We provide os_malloc() and os_free() but not os_realloc(). Add this,
following the usual semantics. Also update os_malloc() to behave correctly
when passed a zero size.

Signed-off-by: Simon Glass <sjg@chromium.org>
Reviewed-by: Heinrich Schuchardt <xypron.glpk@gmx.de>