Slicers for selecting 'all rows' and 'all columns'
Handle empty segemnts in 'check_seg'

Before this change, the second and fourth example failed
while the first and third did not. I think ignoring but
not failing on empty segments is the correct fix.

m([1..2,2..1], 1)

         +1+
   (90)  | |
         +4+
                                                        Type: Matrix(Integer)
m([1..3,3..1], 1)

         +1+
         | |
   (91)  |4|
         | |
         +7+
                                                        Type: Matrix(Integer)
m(1, [1..2,2..1])

   (92)  [1  2]
                                                        Type: Matrix(Integer)
m(1, [1..3,3..1])

   (93)  [1  2  3]
                                                        Type: Matrix(Integer)
Always check for empty segments if segments are in the input