Bytecode + Block decomposition: fix crash with purely forward variables

After simulating a block containing purely forward variables (thus of type
“evaluate backward”), the it_ variable of the evaluator would be left in an
inconsistent state (typically 0, which means that taking the value of a lagged
variable would lead to an invalid read).

By the way, fix a symmetric problem for backward blocks (which could
potentially create a invalid read for purely backward models).

Ref. #1727