Changed order of zero dim / error checking.

Details:
- Updated level-2 and level-3 internal back-ends so that the operation's
  _check() function is called BEFORE any attempt to return early due to
  the presence of zero dimensions. This ordering makes more sense because
  (for example) object dimensions should match even if one of them is
  zero. Previously, a dimension mismatch could result in an early return
  with no error message.
- Updated bli_check_object_buffer() so that NULL buffers result in an
  error only if the object is dimensionally non-empty (i.e., only if both
  of the object's dimensions are non-zero). This allows BLIS operations
  to be performed on dimensionally empty objects (i.e., where at least one
  dimension is zero).
- Updated the error message associated with bli_check_object_buffer()
  to mention the newly relaxed constraint mentioned above, vis-a-vis
  non-zero dimensions.
Merge branch 'master' of github.com:flame/blis