negative small decimal return unexcept result

[lysu]

this issue relates to #6985 but is not the only one question source.

1. What did you do?

mysql> select -0.0000000000000000000000000000000000000000000000000017382578996420603;

2. What did you expect to see?

Field   1:  `-0.0000000000000000000000000000000000000000000000000017382578996420603`
Catalog:    `def`
Database:   ``
Table:      ``
Org_table:  ``
Type:       NEWDECIMAL
Collation:  binary (63)
Length:     71
Max_length: 70
Decimals:   67
Flags:      NOT_NULL BINARY NUM 


+------------------------------------------------------------------------+
| -0.0000000000000000000000000000000000000000000000000017382578996420603 |
+------------------------------------------------------------------------+
| -0.0000000000000000000000000000000000000000000000000017382578996420603 |
+------------------------------------------------------------------------+

3. What did you see instead?

Field   1:  `-0.0000000000000000000000000000000000000000000000000017382578996420603`
Catalog:    `def`
Database:   ``
Table:      ``
Org_table:  ``
Type:       NEWDECIMAL
Collation:  binary (63)
Length:     72
Max_length: 32
Decimals:   67
Flags:      BINARY NUM 


+------------------------------------------------------------------------+
| -0.0000000000000000000000000000000000000000000000000017382578996420603 |
+------------------------------------------------------------------------+
|                                       0.000000000000000000000000000000 |
+------------------------------------------------------------------------+

4. What version of TiDB are you using (tidb-server -V or run select tidb_version(); on TiDB)?

Release Version: v2.1.0-beta-11-g1f6e12cc5
Git Commit Hash: 1f6e12cc55e534bf514eef212f1091a91b8592c9
Git Branch: HEAD
UTC Build Time: 2018-07-04 06:31:26
GoVersion: go version go1.10.1 linux/amd64
Race Enabled: false
TiKV Min Version: 2.1.0-alpha.1-ff3dd160846b7d1aed9079c389fc188f7f5ea13e