IT slide issues

[sezero]

As reported by Thomas Neumann (@neumatho):

I found a module that has some slide issues. It happens at position
4 and 5 at channel 2 and 3. There is a lot of E10 effects and the note
is sliding, but it sounds like it is sliding too far.  I do not know,
if it is the speed or there is missing some bounds checking. Maybe you
can investigate it further.

10000 Marbles.zip

[sezero]

@AliceLR: ideas?