MDEV-11548 Reproducible server crash after the 2nd ALTER TABLE ADD FOREIGN KEY IF NOT EXISTS.

Have to use 'keyname' to check the name uniqueness.

mysql-test/r/alter_table.result

@@ -2076,3 +2076,18 @@ tab1	CREATE TABLE `tab1` (
   PRIMARY KEY (`id`)
 ) ENGINE=MyISAM DEFAULT CHARSET=latin1
 DROP TABLE `tab1`;
+#
+# MDEV-11548 Reproducible server crash after the 2nd ALTER TABLE ADD FOREIGN KEY IF NOT EXISTS
+#
+CREATE TABLE t1 (id INT UNSIGNED NOT NULL PRIMARY KEY);
+CREATE TABLE t2 (id1 INT UNSIGNED NOT NULL);
+ALTER TABLE t2
+ADD FOREIGN KEY IF NOT EXISTS (id1)
+REFERENCES t1 (id);
+ALTER TABLE t2
+ADD FOREIGN KEY IF NOT EXISTS (id1)
+REFERENCES t1 (id);
+Warnings:
+Note	1061	Duplicate key name 'id1'
+DROP TABLE t2;
+DROP TABLE t1;

mysql-test/t/alter_table.test

@@ -1737,3 +1737,22 @@ SHOW CREATE TABLE `tab1`;
 ALTER TABLE `tab1` CHANGE COLUMN v_col `v_col` varchar(128) AS (IF(field11='option1',CONCAT_WS(":","field1",field2,field3,field4,field5,field6,field7,field8,field9,field10), CONCAT_WS(":","field1",field11,field2,field3,field4,field5,field6,field7,field8,field9,field10))) PERSISTENT;
 SHOW CREATE TABLE `tab1`;
 DROP TABLE `tab1`;
+
+--echo #
+--echo # MDEV-11548 Reproducible server crash after the 2nd ALTER TABLE ADD FOREIGN KEY IF NOT EXISTS
+--echo #
+ 
+CREATE TABLE t1 (id INT UNSIGNED NOT NULL PRIMARY KEY);
+CREATE TABLE t2 (id1 INT UNSIGNED NOT NULL);
+ 
+ALTER TABLE t2
+ADD FOREIGN KEY IF NOT EXISTS (id1)
+  REFERENCES t1 (id);
+   
+ALTER TABLE t2
+ADD FOREIGN KEY IF NOT EXISTS (id1)
+REFERENCES t1 (id);
+
+DROP TABLE t2;
+DROP TABLE t1;
+

sql/sql_table.cc

@@ -5943,7 +5943,7 @@ drop_create_field:
         while ((f_key= fk_key_it++))
         {
           if (my_strcasecmp(system_charset_info, f_key->foreign_id->str,
-                key->name.str) == 0)
+                keyname) == 0)
             goto remove_key;
         }
       }