Fixed bug #36488: regexp returns false matches, concatenating

with previous rows.

The WHERE clause containing expression:
  CONCAT(empty_field1, empty_field2, ..., 'literal constant', ...)
    REGEXP 'regular expression'
may return wrong matches.

Optimization of the CONCAT function has been fixed.

mysql-test/r/func_concat.result

@@ -82,3 +82,10 @@ a
 1234562
 x
 drop table t1;
+CREATE TABLE t1 (c1 varchar(100), c2 varchar(100));
+INSERT INTO t1 VALUES ('',''), ('','First'), ('Random','Random');
+SELECT * FROM t1 WHERE CONCAT(c1,' ',c2) REGEXP 'First.*';
+c1	c2
+	First
+DROP TABLE t1;
+# End of 5.0 tests