Fix: maybe-uninitialized warnings in translate-trace-ir-to-ctf-ir.c

Fix: maybe-uninitialized warnings in translate-trace-ir-to-ctf-ir.c

GCC warning example:
  translate-trace-ir-to-ctf-ir.c:929:14: error: ‘struct_fc’ may be used
  uninitialized in this function [-Werror=maybe-uninitialized]
       named_fc = fs_sink_ctf_field_class_struct_borrow_member_by_index(
       ~~~~~~~~~^~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~
      	struct_fc, i);
      	~~~~~~~~~~~~~

Signed-off-by: Francis Deslauriers <francis.deslauriers@efficios.com>
Change-Id: I24ba8095f26cec640e0a24473b4fee05d1c404af
Reviewed-on: https://review.lttng.org/c/babeltrace/+/1032
Reviewed-by: Philippe Proulx <eeppeliteloop@gmail.com>