Fix bug when including xxlarge in $breakpoint-classes

When setting `$breakpoint-classes: (small medium large xlarge xxlarge)` compiling fails, because xxlarge is the last item in the $breakpoints array. The breakpoint mixing will try to grab the value of the next $breakpoints item which results in a null value. The compilation fails at subtracting (1/16) from null: `(Line 62: Invalid null operation: "null minus 0.0625".)`