hv_sock: Check hv_pkt_iter_first_raw()'s return value

The function returns NULL if the ring buffer doesn't contain enough
readable bytes to constitute a packet descriptor.  The ring buffer's
write_index is in memory which is shared with the Hyper-V host, an
erroneous or malicious host could thus change its value and overturn
the result of hvs_stream_has_data().

Signed-off-by: Andrea Parri (Microsoft) <parri.andrea@gmail.com>
Reviewed-by: Michael Kelley <mikelley@microsoft.com>
Reviewed-by: Stefano Garzarella <sgarzare@redhat.com>
Link: https://lore.kernel.org/r/20220428145107.7878-2-parri.andrea@gmail.com
Signed-off-by: Wei Liu <wei.liu@kernel.org>

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