vinyl: fix vy_range_update_compaction_priority hang

Under certain circumstances vy_slice_new() may create an empty slice,
e.g. on range split:

   |------------------ Slice ---------------|
                         |---- Run -----|
                     +
                  split key
   |---- Slice 1 ----||------ Slice 2 ------|
         ^^^^^^^
          Empty

vy_range_update_compaction_priority() uses the size of the last slice in
a range as a base for LSM tree level sizing. If the slice size happens
to be 0, it will simply hang in an infinite loop. Fix this potential
hang by using 1 if the last slice size is 0.