Bug#11766126 59166: ANOTHER DATETIME VALGRIND UNINITIALIZED WARNING

Valgrind warning happens because null values check happens too late
in Item_func_month::val_str(after result string calculation).The fix
is to check null value before result string calculation.

mysql-test/r/func_time.result

@@ -1393,4 +1393,10 @@ SET GLOBAL SQL_MODE=DEFAULT;
 SELECT FORMAT(YEAR(STR_TO_DATE('',GET_FORMAT(TIME,''))),1);
 FORMAT(YEAR(STR_TO_DATE('',GET_FORMAT(TIME,''))),1)
 NULL
+#
+# Bug#11766126  59166: ANOTHER DATETIME VALGRIND UNINITIALIZED WARNING
+#
+SELECT CAST((MONTH(FROM_UNIXTIME(@@GLOBAL.SQL_MODE))) AS BINARY(1025));
+CAST((MONTH(FROM_UNIXTIME(@@GLOBAL.SQL_MODE))) AS BINARY(1025))
+NULL
 End of 5.1 tests

mysql-test/t/func_time.test

@@ -901,4 +901,10 @@ SET GLOBAL SQL_MODE=DEFAULT;
 
 SELECT FORMAT(YEAR(STR_TO_DATE('',GET_FORMAT(TIME,''))),1);
 
+--echo #
+--echo # Bug#11766126  59166: ANOTHER DATETIME VALGRIND UNINITIALIZED WARNING
+--echo #
+
+SELECT CAST((MONTH(FROM_UNIXTIME(@@GLOBAL.SQL_MODE))) AS BINARY(1025));
+
 --echo End of 5.1 tests

sql/item_timefunc.h

@@ -106,8 +106,11 @@ public:
   { DBUG_ASSERT(fixed == 1); return (double) Item_func_month::val_int(); }
   String *val_str(String *str) 
   {
-    str->set(val_int(), &my_charset_bin);
-    return null_value ? 0 : str;
+    longlong nr= val_int();
+    if (null_value)
+      return 0;
+    str->set(nr, &my_charset_bin);
+    return str;
   }
   const char *func_name() const { return "month"; }
   enum Item_result result_type () const { return INT_RESULT; }