Modifying two particle curves sometimes leads to an null reference exception, forcing the user to restart the editor.

Repro steps: (best observed in the attached gif, as the process is a bit convoluted)
1. Enable both Size over lifetime (green) and size by speed (red) curves.
2. In the curve editor, move the leftmost key of the green curve down a bit.
3. Select the red curve and press the minus key to hide it.
4. Reselect the red curve.
5. Select the green curve and press the minus key to hide it.
6. Add a key at the very start of the red curve (where the cursor is almost over the size field and changes to a text cursor.
7. Reselect the green curve so both curves are visible.
8. Drag the green curve's leftmost key up to overlap with the red curve's new key.
9. Null reference exception.

Expected outcome: curve manipulation to now throw a null reference exception.

Notes:
- appears to be a regression; could not repro in Version 5.3.3f1 (910d71450a97)
- I stumbled upon this by adding lots of key frames and playing with the plus and minus signs in the curve inspector. Other combinations might be possible to cause the same issue.
- Editor log attached.