Fix regression in a query involving an aggregate function on a

non-wide decimal column in the HAVING clause.

In buildAggregateColumn(), if an aggregate function (such as avg)
is applied on a non-wide decimal column, we were setting the precision
of the resulting column as -1. This later down in the execution got
converted to 255 as in some cases, precision is stored as uint8_t.
The predicate operations on a DECIMAL column has logic that uses
the wide Decimal::s128value field if precision > 18. This logic incorrectly
used the Decimal::s128value instead of the correct value stored in the
narrow Decimal::value field, since precision of the Decimal column
was 255. The fix is to set the aggregate column precision to
datatypes::INT64MAXPRECISION (18) in buildAggregateColumn() when the
aggregate is applied on a non-wide decimal column.

This commit also partially fixes -Wstrict-aliasing GCC warnings.

dbcon/mysql/ha_mcs_execplan.cpp

@@ -5068,7 +5068,7 @@ ReturnedColumn* buildAggregateColumn(Item* item, gp_walk_info& gwi)
                     {
                         ct.scale += datatypes::MAXSCALEINC4AVG;
                     }
-                    ct.precision = datatypes::IGNOREPRECISION;
+                    ct.precision = datatypes::INT64MAXPRECISION;
                 }
                 ac->resultType(ct);
             }