Fix multiplication of abs() by a negative value. (Bug #11402)

mysql-test/r/func_math.result:
  Add new results
mysql-test/t/func_math.test:
  Add new regression test
sql/item_func.cc:
  Don't set result of abs() to unsigned. Result should still be
  a signed value, even if always positive.

mysql-test/r/func_math.result

@@ -137,3 +137,6 @@ select * from t1;
 round(1, 6)
 1.000000
 drop table t1;
+select abs(-2) * -2;
+abs(-2) * -2
+-4

mysql-test/t/func_math.test

@@ -71,3 +71,8 @@ create table t1 select round(1, 6);
 show create table t1;
 select * from t1;
 drop table t1;
+
+#
+# Bug #11402: abs() forces rest of calculation to unsigned
+#
+select abs(-2) * -2;

sql/item_func.cc

@@ -914,10 +914,7 @@ void Item_func_abs::fix_length_and_dec()
   max_length=args[0]->max_length;
   hybrid_type= REAL_RESULT;
   if (args[0]->result_type() == INT_RESULT)
-  {
     hybrid_type= INT_RESULT;
-    unsigned_flag= 1;
-  }
 }