Close #839, the Shift Times function will now only do something if shifting by a non-zero number of milliseconds, or always if shifting by a number of frames (even zero).

Originally committed to SVN as r3017.

aegisub/src/dialog_shift_times.cpp

@@ -221,6 +221,13 @@ void DialogShiftTimes::OnOK(wxCommandEvent &event) {
 	if (byTime) len = ShiftTime->time.GetMS();
 	else ShiftFrame->GetValue().ToLong(&len);
 
+	if (byTime && len == 0) {
+		// Shift zero milliseconds in time mode
+		// Equivalent to doing nothing at all, so just dismiss
+		EndModal(0);
+		return;
+	}
+
 	// If backwards, invert
 	if (backward) len = -len;