Fixed BUG#2460: Crash wih Stored Procedure and UNION.

mysql-test/r/sp.result:
  Added test case for BUG#2460.
mysql-test/t/sp.test:
  Added test case for BUG#2460.
sql/sql_union.cc:
  When called from stored procedures, tabs might be reset already.

mysql-test/t/sp.test

@@ -1471,6 +1471,52 @@ select @a|
 
 drop procedure bug3863|
 
+#
+# BUG#2460
+#
+--disable_warnings
+drop table if exists t3|
+--enable_warnings
+create table t3 (
+  id int(10) unsigned not null default 0,
+  rid int(10) unsigned not null default 0,
+  msg text not null,
+  primary key (id),
+  unique key rid (rid, id)
+)|
+
+create procedure bug2460_1(in v int)
+begin
+    ( select n0.id from t3 as n0 where n0.id = v )
+  union
+    ( select n0.id from t3 as n0, t3 as n1
+        where n0.id = n1.rid and n1.id = v )
+  union
+    ( select n0.id from t3 as n0, t3 as n1, t3 as n2
+        where n0.id = n1.rid and n1.id = n2.rid and n2.id = v );
+end|
+
+call bug2460_1(2)|
+call bug2460_1(2)|
+insert into t3 values (1, 1, 'foo'), (2, 1, 'bar'), (3, 1, 'zip zap')|
+call bug2460_1(2)|
+call bug2460_1(2)|
+
+create procedure bug2460_2()
+begin
+  drop table if exists t3;
+  create table t3 (s1 int);
+  insert into t3 select 1 union select 1;
+end|
+
+call bug2460_2()|
+call bug2460_2()|
+select * from t3|
+
+drop procedure bug2460_1|
+drop procedure bug2460_2|
+drop table t3|
+
 
 #
 # Some "real" examples