[nrf fromlist] net: lwm2m: Fix null dereference when post-write cb is set

When opaque resources have post-write callback set, but
the write is not a Block-Wise write, there is no block_ctx
and the code causes null pointer dereference when calculating
the offset of the data.

Upstream PR: zephyrproject-rtos/zephyr#73162

Signed-off-by: Seppo Takalo <seppo.takalo@nordicsemi.no>
Signed-off-by: Robert Lubos <robert.lubos@nordicsemi.no>

Author: SeppoTakalo

subsys/net/lib/lwm2m/lwm2m_message_handling.c

@@ -996,10 +996,10 @@ static int lwm2m_write_handler_opaque(struct lwm2m_engine_obj_inst *obj_inst,
 #endif /* CONFIG_LWM2M_ENGINE_VALIDATION_BUFFER_SIZE > 0 */
 
 		if (res->post_write_cb) {
-			ret = res->post_write_cb(obj_inst->obj_inst_id, res->res_id,
-						 res_inst->res_inst_id, data_ptr, len,
-						 last_pkt_block && last_block, opaque_ctx.len,
-						 msg->in.block_ctx->ctx.current);
+			ret = res->post_write_cb(
+				obj_inst->obj_inst_id, res->res_id, res_inst->res_inst_id, data_ptr,
+				len, last_pkt_block && last_block, opaque_ctx.len,
+				(msg->in.block_ctx ? msg->in.block_ctx->ctx.current : 0));
 			if (ret < 0) {
 				return ret;
 			}