bug#54538: (ice-9 suspendable-ports) doesn’t implement the 'escape conversion strategy

‘peek-char’ in (ice-9 suspendable-ports) checks for the 'substitute port
conversion strategy, but it doesn’t check for 'escape:

                (if (eq? (port-conversion-strategy port) 'substitute)
                    (values #\xFFFD buf (+ cur len))
                    (decoding-error "peek-char" port))

and:

             ((eq? (port-conversion-strategy port) 'substitute)
              (values #\xFFFD buf (+ cur prev-input-size)))

This is in 3.0.8.

Ludo’.