Validate M and N dimension inputs to sparse(). * sparse.cc (Fsparse): Use xidx_type_value() to extract an octave_idx_type for M and N inputs or throw an error. Remove call to get_dimensions(). Remove FIXME note about unreachable code. Update error messages to be specific about what is wrong with M or N inputs. * mk-sparse-tst.sh: Remove FIXME notes. Update BIST tests for M and N inputs.

((nil .
      ((c-file-style . "gnu")
       (indent-tabs-mode . nil)
       (fill-column . 72)))
 (change-log-mode . ((indent-tabs-mode . t)))
 (makefile-mode . ((indent-tabs-mode . t))))