MDEV-4436 CHANGE COLUMN IF EXISTS does not work and throws wrong warning.

        Use sql_field->change parameter as the name of the field.

mysql-test/r/alter_table.result

@@ -1899,3 +1899,11 @@ alter table t1 add key if not exists (i);
 Warnings:
 Note	1061	Duplicate key name 'i'
 DROP TABLE t1;
+create table t1 (a int);
+alter table t1 change column if exists a b bigint;
+show create table t1;
+Table	Create Table
+t1	CREATE TABLE `t1` (
+  `b` bigint(20) DEFAULT NULL
+) ENGINE=MyISAM DEFAULT CHARSET=latin1
+DROP TABLE t1;

mysql-test/t/alter_table.test

@@ -1643,3 +1643,11 @@ alter table t1 add key (i);
 alter table t1 add key if not exists (i);
 DROP TABLE t1;
 
+#
+# MDEV-4436 CHANGE COLUMN IF EXISTS does not work and throws wrong warning.
+#
+create table t1 (a int);
+alter table t1 change column if exists a b bigint;
+show create table t1;
+DROP TABLE t1;
+

sql/sql_table.cc

@@ -5309,7 +5309,7 @@ handle_if_exists_options(THD *thd, TABLE *table, Alter_info *alter_info)
       for (f_ptr=table->field; *f_ptr; f_ptr++)
       {
         if (my_strcasecmp(system_charset_info,
-              sql_field->field_name, (*f_ptr)->field_name) == 0)
+              sql_field->change, (*f_ptr)->field_name) == 0)
         {
           break;
         }