fix ALTER TABLE .. DROP CONSTRAINT IF NOT EXISTS

mysql-test/r/check_constraint.result

@@ -84,6 +84,9 @@ t2	CREATE TABLE `t2` (
 ) ENGINE=MyISAM DEFAULT CHARSET=latin1
 alter table t2 drop constraint c;
 ERROR 42000: Can't DROP CONSTRAINT `c`; check that it exists
+alter table t2 drop constraint if exists c;
+Warnings:
+Note	1091	Can't DROP CONSTRAINT `c`; check that it exists
 alter table t2 drop constraint min;
 show create table t2;
 Table	Create Table

mysql-test/t/check_constraint.test

@@ -48,6 +48,7 @@ create table t2 like t1;
 show create table t2;
 --error ER_CANT_DROP_FIELD_OR_KEY
 alter table t2 drop constraint c;
+alter table t2 drop constraint if exists c;
 alter table t2 drop constraint min;
 show create table t2;
 

sql/sql_table.cc

@@ -5880,7 +5880,7 @@ drop_create_field:
       }
       else if (drop->type == Alter_drop::CHECK_CONSTRAINT)
       {
-        for (uint i=0; i < table->s->table_check_constraints; i++)
+        for (uint i=table->s->field_check_constraints; i < table->s->table_check_constraints; i++)
         {
           if (my_strcasecmp(system_charset_info, drop->name,
                             table->check_constraints[i]->name.str) == 0)