Fix: uninitialized variant fc in set_field_refs()

Fix: uninitialized variant fc in set_field_refs()

GCC 8.3.0 emits the following warning:

translate-trace-ir-to-ctf-ir.c:935:16: warning: ‘var_fc’ may be used
uninitialized in this function [-Wmaybe-uninitialized]
     named_fc =
     fs_sink_ctf_field_class_variant_borrow_option_by_index(
                     ^~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~
		           var_fc, i);

I don't think this "condition" is reachable, but it is, in principle
possible. Initializing var_fc to NULL silences this warning.

Signed-off-by: Jérémie Galarneau <jeremie.galarneau@efficios.com>
Change-Id: I1fd8951778e8b3705974324e1cd5d87892c2e418
Reviewed-on: https://review.lttng.org/c/babeltrace/+/1251
Reviewed-by: Francis Deslauriers <francis.deslauriers@efficios.com>