ZKRU/Proto: fix scalar conversion of L2 operations
Ticket amounts are expected to be non-negative by the circuits. The conversion of an L2 operation to scalars must take the absolute value of the amount of the L2 op's header's price.
Ticket amounts are expected to be non-negative by the circuits. The conversion of an L2 operation to scalars must take the absolute value of the amount of the L2 op's header's price.