Fix: maybe-uninitialized warning in create_relative_field_ref()

Fix: maybe-uninitialized warning in create_relative_field_ref()

  translate-trace-ir-to-ctf-ir.c: In function ‘resolve_field_class’:
  translate-trace-ir-to-ctf-ir.c:302:6: error: ‘tgt_fc_name’ may be used
    uninitialized in this function [-Werror=maybe-uninitialized]
      	g_string_assign(tgt_field_ref,
      	^~~~~~~~~~~~~~~~~~~~~~~~~~~~~~
       	 tgt_fc_name);
       	 ~~~~~~~~~~~~
  translate-trace-ir-to-ctf-ir.c:177:14: note: ‘tgt_fc_name’ was declared here
    const char *tgt_fc_name;

Signed-off-by: Francis Deslauriers <francis.deslauriers@efficios.com>
Change-Id: I8d3398a852bf3ca412bb479eae72a81a3959d220
Reviewed-on: https://review.lttng.org/c/babeltrace/+/1031
Reviewed-by: Philippe Proulx <eeppeliteloop@gmail.com>