Fixed the bug mdev-12564.

Here's  what started happening after the patch that fixed
the bug mdev-10454 with query reported for the bug
SELECT * FROM t t1 right JOIN t t2 ON (t2.pk = t1.pk)
  WHERE (t2.i, t2.pk) NOT IN ( SELECT t3.i, t3.i FROM t t3, t t4 )
        AND t1.c = 'foo';
The patch added an implementation of propagate_equal_fields() for
the class Item_row and thus opened the possibility of equal fields
substitutions.
At the prepare stage after setup_conds() called for WHERE condition
had completed the flag of maybe_null of the Item_row object created
for  (t2.i, t2.pk) was set to false, because the maybe_null flags of
both elements were set to false. However the flag of maybe_null for
t1.pk from the ON condition were set to true, because t1 was an inner
table of an outer join.
At the optimization stage the outer join was converted to inner join,
but the maybe_null flags were not corrected and remained the same.
So after the substitution t2.pk/t1.pk. the maybe_null flag for the
row remained false while the maybe_flag for the second element of
the row was true. As a result, when the in-to_exists transformation
was performed for the NOT IN predicate the guards variables were
not created for the elements of the row, but a guard object for
the second element was created. The object were not valid because
it referred to NULL as a guard variable. This ultimately caused
a crash when the expression with the guard was evaluated at the
execution stage.

The patch made sure that the guard objects are not created without
guard variables.

Yet it does not resolve the problem of inconsistent maybe_null flags.
and it might be that the problem will pop op in other pieces of code.
The resolution of this problem is not easy, but the problem should
be resolved in future versions.

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