[FIX] Fix floating point handling in purchase _prepare_order_line_move

Without this better floating point handling, an extra stock move might
be created for zero quantity for some order lines upon PO confirmation,
because qty is equal to something closer to e.g 1.14e-13, but this is
larger than 0, so it creates a stock.move, which gets rounded too late
to 0.0

Closes #3346