[Bug]: alter table rowkey column to varchar(10000) is allowed

[lyukun]

Describe the bug

alter a table columnn to varchar(10000) is allowed in current oceanbase, but that shouldn't be allowed because a varchar may counts for multi bytes and a varchar(10000) may exceed 16KB, which is the max rowkey length.
Environment

• OS Version and CPU Arch(uname -a):
  Linux sqaobnoxdn011124009100.sa128 4.9.151-015.ali3000.alios7.x86_64 SMP Tue Mar 12 19:10:26 CST 2019 x86_64 x86_64 x86_64 GNU/Linux
• OB Version(LD_LIBRARY_PATH=../lib:$LD_LIBRARY_PATH ./observer -V):
  4.3.0

Fast Reproduce Steps(Required)
SQL:
create table t1(c1 int, c2 varchar(1000), c3 int);
create index idx1 on t1(c2, c3);
alter table t1 modify column c3 varchar(10000);

Expected behavior

ERROR 42000: Specified key was too long

Actual Behavior

OK, 0 rows affected in xx seconds.

Additional context