epan: Fix get_display_repr crash on faked nodes

If the node exists but does not have a field info, return an empty
string for the same reason as if the node is faked and doesn't exist.

Fixup 51e0dfa
(which allows faked nodes that exist but have null field info, in order
to preserve the tree shape.)

Fix #20224

epan/proto.c

@@ -7906,8 +7906,10 @@ proto_item_get_display_repr(wmem_allocator_t *scope, proto_item *pi)
 	field_info *fi;
 
 	if (!pi)
-		return "";
+		return wmem_strdup(scope, "");
 	fi = PITEM_FINFO(pi);
+	if (!fi)
+		return wmem_strdup(scope, "");
 	DISSECTOR_ASSERT(fi->hfinfo != NULL);
 	return fvalue_to_string_repr(scope, fi->value, FTREPR_DISPLAY, fi->hfinfo->display);
 }