ctf: silence bogus warning in set_field_refs()

ctf: silence bogus warning in set_field_refs()

gcc 9.1.0 warns that struct_fc my be used uninitialized. The code
flow doesn't allow this to happen, but this warning is annoying/scary
and easily fixed by initializing struct_fc to NULL.

Signed-off-by: Jérémie Galarneau <jeremie.galarneau@efficios.com>
Change-Id: Ib4c702dfc0ca4a29e387b384a1603bdf4634c599
Reviewed-on: https://review.lttng.org/c/babeltrace/+/2028
Reviewed-by: Simon Marchi <simon.marchi@efficios.com>
Tested-by: jenkins <jenkins@lttng.org>