Backport the following from 5.5 to 5.3:

MDEV-4556 Server crashes in SEL_ARG::rb_insert with index_merge+index_merge_sort_union, FORCE INDEX
- merge_same_index_scans() may put the same SEL_ARG tree in multiple result plans.
  make it call incr_refs() on the SEL_ARG trees that it does key_or() on, because 
  key_or(sel_arg_tree_1, sel_arg_tree_2) call may invalidate SEL_ARG trees pointed 
  by sel_arg_tree_1 and sel_arg_tree_2.
    

mysql-test/r/index_merge_myisam.result

@@ -1617,4 +1617,30 @@ GROUP BY 2;
 COUNT(DISTINCT t2.b)	CONCAT(t1.c)
 2	0
 DROP TABLE t1,t2,t3;
+#
+# MDEV-4556 Server crashes in SEL_ARG::rb_insert with index_merge+index_merge_sort_union, FORCE INDEX
+#
+CREATE TABLE t1 (
+pk int,
+code char(2),
+population_rate int,
+area_rate int,
+primary key (pk),
+index (code),
+key (population_rate),
+key (area_rate)
+);
+INSERT INTO t1 VALUES  (1,'WI',20, 23), (2, 'WA', 13, 18);
+EXPLAIN
+SELECT * FROM t1 FORCE INDEX ( PRIMARY, population_rate, area_rate, code ) 
+WHERE pk = 1 OR population_rate = 1 OR ( area_rate IN ( 1,2 ) OR area_rate IS NULL )
+AND (population_rate = 25 OR area_rate BETWEEN 2 AND 25 OR code BETWEEN 'MA' AND 'TX');
+id	select_type	table	type	possible_keys	key	key_len	ref	rows	Extra
+1	SIMPLE	t1	index_merge	PRIMARY,code,population_rate,area_rate	PRIMARY,population_rate,area_rate	4,5,5	NULL	2	Using sort_union(PRIMARY,population_rate,area_rate); Using where
+SELECT * FROM t1 FORCE INDEX ( PRIMARY, population_rate, area_rate, code ) 
+WHERE pk = 1 OR population_rate = 1 OR ( area_rate IN ( 1,2 ) OR area_rate IS NULL )
+AND (population_rate = 25 OR area_rate BETWEEN 2 AND 25 OR code BETWEEN 'MA' AND 'TX');
+pk	code	population_rate	area_rate
+1	WI	20	23
+DROP TABLE t1;
 set optimizer_switch= @optimizer_switch_save;

mysql-test/t/index_merge_myisam.test

@@ -149,5 +149,32 @@ GROUP BY 2;
 
 DROP TABLE t1,t2,t3;
 
+--echo #
+--echo # MDEV-4556 Server crashes in SEL_ARG::rb_insert with index_merge+index_merge_sort_union, FORCE INDEX
+--echo #
+CREATE TABLE t1 (
+  pk int,
+  code char(2),
+  population_rate int,
+  area_rate int,
+  primary key (pk),
+  index (code),
+  key (population_rate),
+  key (area_rate)
+);
+
+INSERT INTO t1 VALUES  (1,'WI',20, 23), (2, 'WA', 13, 18);
+
+EXPLAIN
+SELECT * FROM t1 FORCE INDEX ( PRIMARY, population_rate, area_rate, code ) 
+WHERE pk = 1 OR population_rate = 1 OR ( area_rate IN ( 1,2 ) OR area_rate IS NULL )
+AND (population_rate = 25 OR area_rate BETWEEN 2 AND 25 OR code BETWEEN 'MA' AND 'TX');
+
+SELECT * FROM t1 FORCE INDEX ( PRIMARY, population_rate, area_rate, code ) 
+WHERE pk = 1 OR population_rate = 1 OR ( area_rate IN ( 1,2 ) OR area_rate IS NULL )
+AND (population_rate = 25 OR area_rate BETWEEN 2 AND 25 OR code BETWEEN 'MA' AND 'TX');
+
+DROP TABLE t1;
+
 set optimizer_switch= @optimizer_switch_save;
 

sql/opt_range.cc

@@ -4806,6 +4806,8 @@ TABLE_READ_PLAN *merge_same_index_scans(PARAM *param, SEL_IMERGE *imerge,
       bzero((*changed_tree)->keys,
             sizeof((*changed_tree)->keys[0])*param->keys);
       (*changed_tree)->keys_map.clear_all();
+      key->incr_refs();
+      (*tree)->keys[key_idx]->incr_refs();
       if (((*changed_tree)->keys[key_idx]=
              key_or(param, key, (*tree)->keys[key_idx])))
         (*changed_tree)->keys_map.set_bit(key_idx);