The problem was in an incorrect debug assertion. The expression
used in the failing assertion states that when finding
references matching ORDER BY expressions, there can be only one
reference to a single table. But that does not make any sense,
all test cases for this bug are valid examples with multiple
identical WHERE expressions referencing the same table which
are also present in the ORDER BY list.

Fixed by removing the failing assertion. We also have to take
care of the 'found' counter so that we count multiple
references only once. We rely on this fact later in
eq_ref_table().

mysql-test/r/join.result:
  Added a test case for bug #50335.
mysql-test/t/join.test:
  Added a test case for bug #50335.
sql/sql_select.cc:
  Removing the assertion in eq_ref_table() as it does not make
  any sense. We also have to take care of the 'found' counter so
  that we count multiple references only once. We rely on this
  fact later in eq_ref_table().