ELSE CLAUSE

Problem: During optimization, ZEROFILL values, which includes YEAR
values, may be converted to string constants. However, the CASE
function does not handle switching datatypes after planning, leading
to CASE finding a null pointer instead of its argument.

Item_func_case creates a table of cmp_items, one for each datatype
used in the comparison. This table is created during query planning by
fix_length_and_dec(). During optimization,
Item_field::equal_fields_propagator() converts ZEROFILL numbers to
strings, but the comparison table in Item_func_case is not
updated. Later in optimization, the condition is evaluated as part of
removing trivial comparisons from the query. During evaluation, the
position in the comparison table is found by examining the result
types of the fields to be compared. Since the result type of the
fields have changed, the wrong position in the comparison table is
inferred, and this position contains a null pointer.

This bug is similar to bug#11764818, which affects the IN
function. Item_func_in and Item_func_case are the only functions using
the cmp_items construct.

Fix: Set the cmp_context of WHEN arguments to Item_func_case so that
Item_field::equal_fields_propagator() will not convert them to
strings.