maint: Replace some uses of goto in libinterp. * ls-mat4.cc (read_mat_file_header): Replace "goto data_read_error" with "return -1". * ls-mat4.cc (read_mat_binary_data): Replace "goto data_read_error" with direct call to error. Move variable declarations to point of usage. * ls-mat5.cc (read_mat5_tag, write_mat5_tag): Replace "goto data_read_error" with "return 1". * ls-mat5.cc (save_mat5_binary_element): Replace "goto skip_to_next" with "return true". * ls-oct-binary.cc (read_binary_data): Replace "goto data_read_error" with direct call to error. * mex.cc (valid_key): Replace "goto done" with "return retval". * rand.cc (do_rand): Clarify comment. * pt-binop.cc (tree_binary_expression::rvalue1): Replace "goto done" with "return octave_value (xxx)" * pt-binop.cc (tree_boolean_expression::rvalue1): Replace "goto done" with "return octave_value (xxx)"