MDEV-14696 Server crashes in in prep_alter_part_table on 2nd execution of PS.

        The thd->lex->part_info should be kept intact during PS
        execution. Or the second execution gets that modified part_info.
        Let's modify ths->work_part_info instead.

mysql-test/r/partition.result

@@ -2634,3 +2634,14 @@ alter table t1 drop partition if exists p5;
 Warnings:
 Note	1507	Error in list of partitions to DROP
 DROP TABLE t1;
+CREATE TABLE t1 (a INT) ENGINE=MyISAM PARTITION BY RANGE(a) (PARTITION p1 VALUES LESS THAN (0));
+ALTER TABLE t1 ADD PARTITION (PARTITION p2 VALUES LESS THAN (1));
+PREPARE stmt FROM 'ALTER TABLE t1 ADD PARTITION IF NOT EXISTS (PARTITION p2 VALUES LESS THAN (2))';
+EXECUTE stmt;
+Warnings:
+Note	1517	Duplicate partition name p2
+EXECUTE stmt;
+Warnings:
+Note	1517	Duplicate partition name p2
+DEALLOCATE PREPARE stmt;
+DROP TABLE t1;

mysql-test/t/partition.test

@@ -2884,3 +2884,16 @@ alter table t1 drop partition if exists p5;
 
 DROP TABLE t1;
 
+#
+# MDEV-14696 Server crashes in in prep_alter_part_table on 2nd execution of PS.
+#
+
+CREATE TABLE t1 (a INT) ENGINE=MyISAM PARTITION BY RANGE(a) (PARTITION p1 VALUES LESS THAN (0));
+ALTER TABLE t1 ADD PARTITION (PARTITION p2 VALUES LESS THAN (1));
+PREPARE stmt FROM 'ALTER TABLE t1 ADD PARTITION IF NOT EXISTS (PARTITION p2 VALUES LESS THAN (2))';
+EXECUTE stmt;
+EXECUTE stmt;
+ 
+DEALLOCATE PREPARE stmt;
+DROP TABLE t1;
+

sql/sql_partition.cc

@@ -4713,7 +4713,12 @@ uint prep_alter_part_table(THD *thd, TABLE *table, Alter_info *alter_info,
     DBUG_RETURN(TRUE);
   }
 
+  /*
+    One of these is done in handle_if_exists_option():
         thd->work_part_info= thd->lex->part_info;
+      or
+        thd->work_part_info= NULL;
+  */
 
   if (thd->work_part_info &&
       !(thd->work_part_info= thd->work_part_info->get_clone()))

sql/sql_table.cc

@@ -5997,6 +5997,7 @@ handle_if_exists_options(THD *thd, TABLE *table, Alter_info *alter_info)
   
 #ifdef WITH_PARTITION_STORAGE_ENGINE
   partition_info *tab_part_info= table->part_info;
+  thd->work_part_info= thd->lex->part_info;
   if (tab_part_info && thd->lex->check_exists)
   {
     /* ALTER TABLE ADD PARTITION IF NOT EXISTS */
@@ -6015,7 +6016,7 @@ handle_if_exists_options(THD *thd, TABLE *table, Alter_info *alter_info)
                 ER_SAME_NAME_PARTITION, ER(ER_SAME_NAME_PARTITION),
                 pe->partition_name);
             alter_info->flags&= ~Alter_info::ALTER_ADD_PARTITION;
-            thd->lex->part_info= NULL;
+            thd->work_part_info= NULL;
             break;
           }
         }