Fix: may-uninitialized warning in trace-ir-metadata-field-class-copy.c

Fix: may-uninitialized warning in trace-ir-metadata-field-class-copy.c

GCC warning:
  In file included from ../../../include/babeltrace/common-internal.h:30:0,
                   from trace-ir-metadata-field-class-copy.c:31:
  trace-ir-metadata-field-class-copy.c: In function ‘copy_field_class_content_internal’:
  ../../../include/babeltrace/trace-ir/field-class-const.h:196:3: error:
  ‘out_tag_field_class’ may be used uninitialized in this function
  [-Werror=maybe-uninitialized]
     bt_field_class_put_ref(_var);  \
     ^~~~~~~~~~~~~~~~~~~~~~
  trace-ir-metadata-field-class-copy.c:421:18: note: ‘out_tag_field_class’ was declared here
    bt_field_class *out_tag_field_class;

Signed-off-by: Francis Deslauriers <francis.deslauriers@efficios.com>
Change-Id: I73c6057f9795b153f95c1287f3f3f8e529328fbf
Reviewed-on: https://review.lttng.org/c/babeltrace/+/1033
Reviewed-by: Philippe Proulx <eeppeliteloop@gmail.com>