bgmac: fix a missing check for build_skb

when build_skb failed, it may occure a NULL pointer.
So add a 'NULL check' for it.

Signed-off-by: Weidong Wang <wangweidong1@huawei.com>
Signed-off-by: David S. Miller <davem@davemloft.net>

drivers/net/ethernet/broadcom/bgmac.c

@@ -466,6 +466,11 @@ static int bgmac_dma_rx_read(struct bgmac *bgmac, struct bgmac_dma_ring *ring,
 			len -= ETH_FCS_LEN;
 
 			skb = build_skb(buf, BGMAC_RX_ALLOC_SIZE);
+			if (unlikely(skb)) {
+				bgmac_err(bgmac, "build_skb failed\n");
+				put_page(virt_to_head_page(buf));
+				break;
+			}
 			skb_put(skb, BGMAC_RX_FRAME_OFFSET +
 				BGMAC_RX_BUF_OFFSET + len);
 			skb_pull(skb, BGMAC_RX_FRAME_OFFSET +