net: lwm2m: Add missing NULL pointer check in shell

In case lwm2m_get_engine_obj_field() fails to find a corresponding
object field when iterating resource instances, simply skip that
resource instance when printing object instance contents.

Signed-off-by: Robert Lubos <[email protected]>

Author: rlubos

subsys/net/lib/lwm2m/lwm2m_shell.c

@@ -735,6 +735,10 @@ static int print_object_instance(const struct shell *sh, struct lwm2m_engine_obj
 					lwm2m_get_engine_obj_field(obj, re->res_id);
 				char path[LWM2M_MAX_PATH_STR_SIZE];
 
+				if (field == NULL) {
+					continue;
+				}
+
 				if (re->multi_res_inst) {
 					snprintf(path, sizeof(path), "%hu/%hu/%hu/%hu", obj->obj_id,
 						 oi->obj_inst_id, re->res_id, ri->res_inst_id);