Fix: ctf-writer: stream-class.c: Dereference after null check

Coverity reported the following:
  CID 1401231 (#1 of 1): Dereference after null check (FORWARD_NULL)8.
  var_deref_model: Passing null pointer ft_copy to
  bt_ctf_field_type_common_integer_set_mapped_clock_class_no_check_frozen,
  which dereferences it

Reported-by: Coverity - 1401231 Dereference after null check (FORWARD_NULL)
Signed-off-by: Francis Deslauriers <francis.deslauriers@efficios.com>
Change-Id: I32a15f0ff927c8b987e785a37596e9c56b628de5
Reviewed-on: https://review.lttng.org/c/babeltrace/+/2049
Tested-by: jenkins <jenkins@lttng.org>
Reviewed-by: Simon Marchi <simon.marchi@efficios.com>