[Sortable] Fixes issue with off by one error when incrementing sort position field

BUG: Found issue with incrementing position field with Sortable

The calculation of the max sort position is off by one. This means when incrementing the last object's sort position or setting an out of bounds sort position on any of the other objects it is possible to get a non-sequential sort number.
 
E.g.
 
list[0]
list[1]
list[2]
list[3] <- incrementing the sort position of this will result in [0,1,2,4]