fix bug in SourceSelection: don't set s to temporary values (#2313)

- If we were on OLED, and last allowable modulation source was
  highlighted, but there were non-allowed ones after it, trying to
  scroll further would bail out with with SourceSelection::s set
  to a non-allowed value even though we had an allowed on selected.

  Solution is to set s only after we've found a legal value: in
  event of the bailout we retain the previous legal value.

- fixes #2312

src/deluge/gui/menu_item/source_selection.cpp

@@ -200,10 +200,9 @@ void SourceSelection::selectEncoderAction(int32_t offset) {
 			newValue = ((newValue % kNumPatchSources) + kNumPatchSources) % kNumPatchSources;
 		}
 
-		s = sourceMenuContents[newValue];
-
-	} while (!sourceIsAllowed(s));
+	} while (!sourceIsAllowed(sourceMenuContents[newValue]));
 
+	s = sourceMenuContents[newValue];
 	this->setValue(newValue);
 
 	if (display->haveOLED()) {