MDEV-37740 LOCATE(X,Y,NULL) is not NULL

LOCATE is documented to return NULL if any of its arguments is NULL

Author: vuvova

mysql-test/main/func_str.result

@@ -5623,5 +5623,9 @@ DROP TABLE t2;
 DROP TABLE t1;
 SET sql_mode=DEFAULT;
 #
-# End of 10.11 tests
+# MDEV-37740 LOCATE(X,Y,NULL) is not NULL
 #
+select locate(1,2,NULL);
+locate(1,2,NULL)
+NULL
+# End of 10.11 tests

mysql-test/main/func_str.test

@@ -2573,5 +2573,8 @@ DROP TABLE t1;
 SET sql_mode=DEFAULT;
 
 --echo #
---echo # End of 10.11 tests
+--echo # MDEV-37740 LOCATE(X,Y,NULL) is not NULL
 --echo #
+select locate(1,2,NULL);
+
+--echo # End of 10.11 tests

sql/item_func.cc

@@ -3247,7 +3247,10 @@ longlong Item_func_locate::val_int()
     start0= start= args[2]->val_int();
 
     if ((start <= 0) || (start > a->length()))
+    {
+      null_value= args[2]->is_null();
       return 0;
+    }
     start0--; start--;
 
     /* start is now sufficiently valid to pass to charpos function */