BUG#15820 - Can create table with 1 partition, then doing ALTER .. ADD PARTITION crashes

It is ok for a partitioned table to have only 1 partition.  This test is just to show that
doing the ALTER TABLE ADD PARTITION doesn't crash the server.

partition_mgm_err.result:
  Added result block for test of bug #15820
partition_mgm_err.test:
  Added test block for bug #15820

mysql-test/r/partition_mgm_err.result

@@ -140,3 +140,5 @@ t1	CREATE TABLE `t1` (
   `a` int(11) default NULL
 ) ENGINE=MyISAM DEFAULT CHARSET=latin1 PARTITION BY KEY (a) PARTITIONS 2 
 DROP TABLE t1;
+CREATE TABLE t1 (a INT) PARTITION BY HASH(a);
+ALTER TABLE t1 ADD PARTITION PARTITIONS 4;

mysql-test/t/partition_mgm_err.test

@@ -210,3 +210,9 @@ SHOW CREATE TABLE t1;
 ALTER TABLE t1 PARTITION BY KEY(a) PARTITIONS 2;
 SHOW CREATE TABLE t1;
 DROP TABLE t1;
+
+#
+#BUG 15820 create table with 1 partition, doing ALTER TABLE ADD PARTITION fails
+#
+CREATE TABLE t1 (a INT) PARTITION BY HASH(a);
+ALTER TABLE t1 ADD PARTITION PARTITIONS 4;