Type overflow in DocumentMerger when scrolling a large file

When file is quite large, and scrolling is needed, org.eclipse.compare.internal.merge.DocumentMerger.realToVirtualPosition(char, int) method can have type overflow when multiplying (see 1022 line). This gives sometimes negative number and results in wrong scrolling position for example.

In similar method virtualToRealPosition this doesnot seem to be a problem since multiplication of numbers happen after division and overflow is unlikely to happen, so may be similar approach should be used to realToVirtualPosition function