Fix ValidateDateTime: check day instead month

Found with PVS-Studio static analyser, see 
https://www.viva64.com/en/b/0550/

src/google/protobuf/stubs/time.cc

@@ -80,9 +80,9 @@ bool ValidateDateTime(const DateTime& time) {
     return false;
   }
   if (time.month == 2 && IsLeapYear(time.year)) {
-    return time.month <= kDaysInMonth[time.month] + 1;
+    return time.day <= kDaysInMonth[time.month] + 1;
   } else {
-    return time.month <= kDaysInMonth[time.month];
+    return time.day <= kDaysInMonth[time.month];
   }
 }