MDEV-36220 Correct length in memcpy saving and restoring found NULL record in loose index scan of min

Use reclength because rec_buff_length is the actual reclength with
padding, whose use could cause ASAN unknown-crash, presumably caused
by memory violation.

mysql-test/main/group_min_max.result

@@ -4359,5 +4359,17 @@ MAX(b)
 3
 drop table t1;
 #
+# MDEV-36220 ASAN unknown-crash in loose index scan of MIN with IS NULL
+#
+CREATE TABLE t1 (a int, b int, KEY (a, b));
+insert into t1 values (4, NULL), (1, 14), (4, 3);
+SELECT MIN(b) FROM t1 WHERE b = 3 OR b IS NULL GROUP BY a;
+MIN(b)
+3
+SELECT MIN(b) FROM t1 WHERE b IS NULL GROUP BY a;
+MIN(b)
+NULL
+drop table t1;
+#
 # End of 10.11 tests
 #

mysql-test/main/group_min_max.test

@@ -2030,6 +2030,16 @@ if ($have_debug) {
 
 drop table t1;
 
+--echo #
+--echo # MDEV-36220 ASAN unknown-crash in loose index scan of MIN with IS NULL
+--echo #
+
+CREATE TABLE t1 (a int, b int, KEY (a, b));
+insert into t1 values (4, NULL), (1, 14), (4, 3);
+SELECT MIN(b) FROM t1 WHERE b = 3 OR b IS NULL GROUP BY a;
+SELECT MIN(b) FROM t1 WHERE b IS NULL GROUP BY a;
+drop table t1;
+
 --echo #
 --echo # End of 10.11 tests
 --echo #

sql/opt_range.cc

@@ -16017,7 +16017,7 @@ int QUICK_GROUP_MIN_MAX_SELECT::next_min_in_range()
         Remember this key, and continue looking for a non-NULL key that
         satisfies some other condition.
       */
-      memcpy(tmp_record, record, head->s->rec_buff_length);
+      memcpy(tmp_record, record, head->s->reclength);
       found_null= TRUE;
       continue;
     }
@@ -16057,7 +16057,7 @@ int QUICK_GROUP_MIN_MAX_SELECT::next_min_in_range()
   */
   if (found_null && result)
   {
-    memcpy(record, tmp_record, head->s->rec_buff_length);
+    memcpy(record, tmp_record, head->s->reclength);
     result= 0;
   }
   return result;