Fixed bug #66242 (don't assume char is signed)

This fixes a bug in libmagic where a cast to 'char' is assumed to result
in sign extension to occur. However, unqualified 'char' is unsigned on
architectures such as ARM, so the cast needs to be to 'signed char'
explicitly.

Signed-off-by: Ard Biesheuvel <ard.biesheuvel@linaro.org>