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

Item_func_in 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_in is not
updated. During execution, 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.

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