ctf: silence bogus warnings in create_relative_field_ref()

ctf: silence bogus warnings in create_relative_field_ref()

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

Signed-off-by: Jérémie Galarneau <jeremie.galarneau@efficios.com>
Change-Id: I554affb70275f60eff08134f80b33bc4cefbc91f
Reviewed-on: https://review.lttng.org/c/babeltrace/+/2027
Reviewed-by: Simon Marchi <simon.marchi@efficios.com>
Tested-by: jenkins <jenkins@lttng.org>