**Describe the bug**
Performing `sumMap` function with decimal values does not sum properly. looks like an overflow.

**Does it reproduce on recent release?**
[The list of releases](https://github.com/ClickHouse/ClickHouse/blob/master/utils/list-versions/version_date.tsv)
Yes. tried on latest `21.1.2.15` as well as `20.12.4.5`

**How to reproduce**
```sql
CREATE TABLE sum_map(
    statusMap Nested(
        goal_id UInt16,
        revenue Decimal32(5)
    )
) ENGINE = Log;

INSERT INTO sum_map VALUES
    ([1, 2, 3], [10.5, 10.5, 10.5]),
    ([3, 4, 5], [10.5, 10.5, 10.5]),
    ([4, 5, 6], [10.5, 10.5, 10.5]),
    ([6, 7, 8], [10.5, 10.5, 10.5]);

SELECT
    sumMap(statusMap.goal_id, statusMap.revenue)
FROM sum_map;
```

**Expected behavior**
The decimal values in the nested revenue column should be summed properly. The select above leads to:
```
+---------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------+
| sumMap(statusMap.goal_id, statusMap.revenue)                                                                                                                                                                                                                                                                                    |
+---------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------+
| ([1,2,3,4,5,6,7,8],[1521106780904990803940045903404371.81840,-305731560536066622954267316264999.30736,25802410697087604800041730231.52928,25802360332470575101123994625.01152,25802360332494186933538342851.08000,25802360332517798765952691077.14848,25802360332694887509060302762.16208,25802360332742111173888999214.29904]) |
+---------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------+
```

**note** when writing this issue I accidentally [submitted pre-maturely](https://github.com/ClickHouse/ClickHouse/issues/19380) and closed that one.
extern const int LOGICAL_ERROR;
}
template <typename T>
struct AggregateFunctionMapData
{
    
    std::map<T, Array> merged_maps;
};
/** Aggregate function, that takes at least two arguments: keys and values, and as a result, builds a tuple of of at least 2 arrays -
  * ordered keys and variable number of argument values aggregated by corresponding keys.
  *
  * sumMap function is the most useful when using SummingMergeTree to sum Nested columns, which name ends in "Map".
  *
  * Example: sumMap(k, v...) of:
  *  k           v
  *  [1,2,3]     [10,10,10]
  *  [3,4,5]     [10,10,10]
  *  [4,5,6]     [10,10,10]
  *  [6,7,8]     [10,10,10]
  *  [7,5,3]     [5,15,25]
  *  [8,9,10]    [20,20,20]
  * will return:
  *  ([1,2,3,4,5,6,7,8,9,10],[10,10,45,20,35,20,15,30,20,20])
  *
  * minMap and maxMap share the same idea, but calculate min and max correspondingly.
  *
  * NOTE: The implementation of these functions are "amateur grade" - not efficient and low quality.
  */
template <typename T, typename Derived, typename Visitor, bool overflow, bool tuple_argument, bool compact>
class AggregateFunctionMapBase : public IAggregateFunctionDataHelper<