openamp: add assert when get tx buffer failed

add assert for judge it can't get buffer after wait RPMSG_TICK_COUNT time,
and in this case, it can be jugded as logic problem directly

Signed-off-by: Jiuzhu1 Dong <dongjiuzhu1@xiaomi.com>
Signed-off-by: Yongrong Wang <wangyongrong@xiaomi.com>

Author: Donny9

lib/rpmsg/rpmsg_virtio.c

@@ -390,6 +390,8 @@ static void *rpmsg_virtio_get_tx_payload_buffer(struct rpmsg_device *rdev,
 		}
 	}
 
+	metal_assert(!wait || rp_hdr);
+
 	if (!rp_hdr)
 		return NULL;