fix DIFFICULTY_INVALID used as a valid value

stepmania/src/ScreenSelectMusic.cpp

@@ -1349,6 +1349,8 @@ void ScreenSelectMusic::SwitchToPreferredDifficulty()
 					break;
 				}
 
+				if( GAMESTATE->m_PreferredDifficulty[pn] != DIFFICULTY_INVALID )
+				{
 					int iDiff = abs(m_vpSteps[i]->GetDifficulty() - GAMESTATE->m_PreferredDifficulty[pn]);
 
 					if( iCurDifference == -1 || iDiff < iCurDifference )
@@ -1357,6 +1359,7 @@ void ScreenSelectMusic::SwitchToPreferredDifficulty()
 						iCurDifference = iDiff;
 					}
 				}
+			}
 
 			CLAMP( iSelection, 0, m_vpSteps.size()-1 );
 		}