Should not add an index of length 0 to columns of type Blob, text, etc.

[zimulala]

Bug Report

Please answer these questions before submitting your issue. Thanks!

1. What did you do?
   If possible, provide a recipe for reproducing the error.

create table t(a blob(10), index(a(0)));

2. What did you expect to see?

mysql> create table t(a blob(10), index(a(0)));
ERROR 1391 (HY000): Key part 'a' length cannot be 0

3. What did you see instead?

tidb> create table t(a blob(10), index(a(0)));
Query OK, 0 rows affected (0.01 sec)

4. What version of TiDB are you using (tidb-server -V or run select tidb_version(); on TiDB)?
   ["Release Version"=v3.0.0-rc.1-264-gcb23b524a]
   ["Git Commit Hash"=cb23b524accce9ff1d1168acb0b5a102f3e7a22c]

[tiancaiamao]

Would you like to handle it? if so, you may assign this issue to yourself. @zimulala

[zimulala]

We want it to return an error.

[Rustin170506]

I am working on it.🤓

[zimulala]

@Rustin-Liu
Thank you for your participation！