ipsec: be careful of non existing mac headers

Niccolo Belli reported ipsec crashes in case we handle a frame without
mac header (atm in his case)

Before copying mac header, better make sure it is present.

Bugzilla reference:  https://bugzilla.kernel.org/show_bug.cgi?id=42809



Reported-by: Niccolò Belli <darkbasic@linuxsystems.it>
Tested-by: Niccolò Belli <darkbasic@linuxsystems.it>
Signed-off-by: Eric Dumazet <eric.dumazet@gmail.com>
Signed-off-by: David S. Miller <davem@davemloft.net>

CRs-Fixed: 344300
Change-Id: Ic2d8f3f6b90233d6e87741373c33ef510b35aaa5
Signed-off-by: Jin Hong <jinh@codeaurora.org>