Interpolation enters junk data

[jpcima]

On ef71afc, sometimes interpolating will insert bad values into the pattern.
This seems only happening if the interpolated range is long enough, a shorter one is fine.

Example

Steps

1. start with the empty song
2. at the volume column at row 00, enter 60
3. at the volume column at row 30, enter 7F
4. select the volume column 00 to 30 using shift key and arrows
5. press ctrl+g for interpolation