MDEV-5743 Server crashes in mysql_alter_table on an attempt to add a primary key to InnoDB table

mysql-test/r/alter_table_trans.result

@@ -16,3 +16,11 @@ insert t1 values (repeat('3', 8193),3,1,1);
 ALTER TABLE t1 ADD PRIMARY KEY (col4(10)) , ADD UNIQUE KEY uidx (col3);
 ERROR 23000: Duplicate entry '1' for key 'uidx'
 DROP TABLE t1;
+CREATE TABLE t1 (a INT) ENGINE = InnoDB;
+INSERT INTO t1 VALUES (2);
+ALTER TABLE t1 ADD PRIMARY KEY (a);
+ALTER TABLE t1 DROP PRIMARY KEY;
+INSERT INTO t1 VALUES (2);
+ALTER TABLE t1 ADD PRIMARY KEY (a);
+ERROR 23000: Duplicate entry '2' for key 'PRIMARY'
+DROP TABLE t1;

mysql-test/t/alter_table_trans.test

@@ -32,4 +32,16 @@ insert t1 values (repeat('3', 8193),3,1,1);
 ALTER TABLE t1 ADD PRIMARY KEY (col4(10)) , ADD UNIQUE KEY uidx (col3);
 DROP TABLE t1;
 
+#
+# MDEV-5743 Server crashes in mysql_alter_table on an attempt to add a primary key to InnoDB table
+#
+
+CREATE TABLE t1 (a INT) ENGINE = InnoDB;
+INSERT INTO t1 VALUES (2);
+ALTER TABLE t1 ADD PRIMARY KEY (a);
+ALTER TABLE t1 DROP PRIMARY KEY;
+INSERT INTO t1 VALUES (2);
+--error ER_DUP_ENTRY
+ALTER TABLE t1 ADD PRIMARY KEY (a);
+DROP TABLE t1;
 

sql/sql_table.cc

@@ -6908,7 +6908,7 @@ bool mysql_alter_table(THD *thd,char *new_db, char *new_name,
           if (is_PK(key_info))
           {
             merged[0]= key_info[0];
-            if (is_PK(table->key_info))
+            if (table->key_info && is_PK(table->key_info))
             {
               old_cnt--;
               table->key_info++;