Attest: fix buffer value check in token test code

q_useful_buf_is_value() returns a value equal to or greater than 0
in every case. If all bytes match (with 0 in this case), it will return
SIZE_MAX.

Signed-off-by: David Vincze <david.vincze@arm.com>
Change-Id: I7baef99ab50d398a70b87b1000e0b14d7a0ec934
1 file changed