Fixed dummy token getting the wrong character offset.

The dummy token is added to the index at the end of each document, so
there's a position to store XML tags that occur after the last word. The
dummy token accidentally always got 0 for the character offset, which
caused problems elsewhere.
Made sure old indexing bug can't cause a crash anymore.

The bug was that the dummy token added at the end of each document (see
previous commit) would always get character position 0.
Searcher.getCharacterOffsets() now makes sure this incorrect value is
never used, instead using the character offset of the previous token in
this case (which is correct).