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.


mysql-test/r/select.result:
  Added test for BUG#13597
mysql-test/t/select.test:
  Added test for BUG#13597
sql/table.cc:
  - test whether a table reference is a right join by testing the
    rigth join operand (first in the list of operands), and not
    the table reference that represents the join itself.
  - clearer comments