Validate input UCI moves

Running following command:

  position startpos moves e1e8

Makes SF to assert in debug mode in do_move() but to accept
bad input and continue in release mode where probably it is
going to crash little later.

So validate input before to feed do_move().

Suggestion by Yakovlev Vadim.

No functional change.

Signed-off-by: Marco Costalba <mcostalba@gmail.com>