fdtdump.c: make sure size_t argument to memchr is always unsigned.

CID 132817 (#1 of 1): Integer overflowed argument (INTEGER_OVERFLOW)
15. overflow_sink: Overflowed or truncated value (or a value computed from an overflowed or truncated value) endp - p - 4L used as critical argument to function.

Signed-off-by: Jean-Christophe Dubois <jcd@tribudubois.net>
Signed-off-by: David Gibson <david@gibson.dropbear.id.au>