Fix for BUG#13597 - columns in ON condition not resolved if references a table in a nested right join.

The problem was in that when finding the last table reference in a nested join tree,
the procedure doing the iteration over the right-most branches of a join tree
was testing for RIGHT JOINs the table reference that represents the join, and not
the second operand of the JOIN. Currently the information whether a join is LEFT/RIGHT
is stored not on the join object itself, but on one of its operands.