cmocka: Fix assert_memory_equal() display

The %x specifier expects an unsigned argument. If char is signed,
cmocka_print_error() may incorrectly display values sign-extended. To
fix this, use an unsigned char and the corresponding format specifier
(%hhx).

Signed-off-by: Joseph Sutton <josephsutton@catalyst.net.nz>
Reviewed-by: Andreas Schneider <asn@cryptomilk.org>
(backported from commit 2206c225)