Fix bug for 'eval' command in terminal

The 'eval' debugging command in Terminal did not initialize the Eval::Contempt
variable, leading to random output during debugging sessions (normal search
was unaffected by the bug).

Example of session where the two 'eval' commands should give the same output,
but did not:

./stockfish
position startpos
d
eval
go depth 20
d
eval

The bug is fixed by initializing Eval::Contempt to SCORE_ZERO in Eval::trace

No functional change.