Changed treatment of NULL object buffers.

Details:
- Relaxed the constraint in bli_obj_attach_buffer_check(), which required
  the buffer address being attached to be non-NULL. This is acceptable
  because the user was already able to create and use objects with NULL
  buffers (via bli_obj_create_without_buffer(), which initializes the
  buffer to NULL).
- Inserted calls to newly defined function, bli_check_object_buffer(),
  into nearly all operations' _check() or _int_check() functions. This
  allows BLIS to abort peacefully if a computational routine is called
  with an object containing a NULL buffer. By contrast, under such
  conditions, BLAS would typically fail with a segmentation fault.
- Within operation front-ends, moved the calls to _check()/_int_check()
  so that zero dimensions are checked first (and if found, execution
  returns with trivial or no computation). This resolves issue #7. Thanks
  to Jack Poulson for reporting this bug.