virtio_net: fix return value check in receive_mergeable()

[ Upstream commit 03e9f8a05bce7330bcd9c5cc54c8e42d0fcbf993 ]

The function virtqueue_get_buf_ctx() could return NULL, the return
value 'buf' need to be checked with NULL, not value 'ctx'.

Signed-off-by: Yunjian Wang <wangyunjian@huawei.com>
Signed-off-by: Michael S. Tsirkin <mst@redhat.com>
Signed-off-by: Sasha Levin <alexander.levin@verizon.com>
Signed-off-by: Greg Kroah-Hartman <gregkh@linuxfoundation.org>