Fix for bug #13044: BIT_COUNT with NULL values.

sql/item_func.cc:
  Fix for bug #13044: BIT_COUNT with NULL values.
  Always set null_value.

mysql-test/r/func_op.result

@@ -25,3 +25,14 @@ select -1 >> 0, -1 << 0;
 select -1 >> 1, -1 << 1;
 -1 >> 1	-1 << 1
 9223372036854775807	18446744073709551614
+drop table if exists t1,t2;
+create table t1(a int);
+create table t2(a int, b int);
+insert into t1 values (1), (2), (3);
+insert into t2 values (1, 7), (3, 7);
+select t1.a, t2.a, t2.b, bit_count(t2.b) from t1 left join t2 on t1.a=t2.a;
+a	a	b	bit_count(t2.b)
+1	1	7	3
+2	NULL	NULL	NULL
+3	3	7	3
+drop table t1, t2;

mysql-test/t/func_op.test

@@ -14,3 +14,15 @@ select  1 | -1,  1 ^ -1,  1 & -1;
 select  0 | -1,  0 ^ -1,  0 & -1;
 select -1 >> 0, -1 << 0;
 select -1 >> 1, -1 << 1;
+
+#
+# Bug 13044: wrong bit_count() results
+#
+
+drop table if exists t1,t2;
+create table t1(a int);
+create table t2(a int, b int);
+insert into t1 values (1), (2), (3);
+insert into t2 values (1, 7), (3, 7);
+select t1.a, t2.a, t2.b, bit_count(t2.b) from t1 left join t2 on t1.a=t2.a;
+drop table t1, t2;

sql/item_func.cc

@@ -1138,11 +1138,8 @@ longlong Item_func_find_in_set::val_int()
 longlong Item_func_bit_count::val_int()
 {
   ulonglong value= (ulonglong) args[0]->val_int();
-  if (args[0]->null_value)
-  {
-    null_value=1; /* purecov: inspected */
+  if ((null_value= args[0]->null_value))
     return 0; /* purecov: inspected */
-  }
   return (longlong) my_count_bits(value);
 }