MDEV-13535 Query on MyISAM table corrupts the table

don't assume that the table always has a primary key,
head->s->primary_key might be MAX_KEY, which won't start keyread.

mysql-test/include/index_merge1.inc

@@ -64,6 +64,8 @@ explain
 select * from t0 where key1 < 3 or key2 > 1020;
 select * from t0 where key1 < 3 or key2 > 1020;
 
+select * from t0 where key1=1022; # MDEV-13535 no-key-read select after keyread
+
 explain select * from t0 where key1 < 3 or key2 <4;
 
 explain

mysql-test/r/index_merge_myisam.result

@@ -34,6 +34,9 @@ key1	key2	key3	key4	key5	key6	key7	key8
 1022	1022	1022	1022	1022	1022	1022	2
 1023	1023	1023	1023	1023	1023	1023	1
 1024	1024	1024	1024	1024	1024	1024	0
+select * from t0 where key1=1022;
+key1	key2	key3	key4	key5	key6	key7	key8
+1022	1022	1022	1022	1022	1022	1022	2
 explain select * from t0 where key1 < 3 or key2 <4;
 id	select_type	table	type	possible_keys	key	key_len	ref	rows	Extra
 1	SIMPLE	t0	index_merge	i1,i2	i1,i2	4,4	NULL	7	Using sort_union(i1,i2); Using where

sql/opt_range.cc

@@ -10889,13 +10889,13 @@ int read_keys_and_merge_scans(THD *thd,
   DBUG_ENTER("read_keys_and_merge");
 
   /* We're going to just read rowids. */
-  head->file->ha_start_keyread(head->s->primary_key);
   head->prepare_for_position();
 
   cur_quick_it.rewind();
   cur_quick= cur_quick_it++;
   bool first_quick= TRUE;
   DBUG_ASSERT(cur_quick != 0);
+  head->file->ha_start_keyread(cur_quick->index);
   
   /*
     We reuse the same instance of handler so we need to call both init and