Fix openutau crashing when trying to edit the preutterence of an invalid phoneme

[oxygen-dioxide]

Steps to reproduce:

• Launch OpenUtau. Create a project. Create a track. Create a part. Open piano roll.
• Create 3 notes. The first and the last note are valid, while the second one is invalid
  
• Drag the invalid phonemes's timing bar somewhere, move the preutter point of the next phoneme onto it
• Click here
  

Before this fix, OpenUtau will crash.