divide error in nf_nat_l4proto_unique_tuple

Hello,

syzbot has tested the proposed patch and the reproducer did not trigger  
crash:

Reported-and-tested-by:  
syzbot+8012e198bd037f4871e5@syzkaller.appspotmail.com

Note: the tag will also help syzbot to understand when the bug is fixed.

Tested on net commit
07a2e1cf398187814b405665b19d36425ec7a962 (Mon Feb 12 17:20:11 2018 +0000)
net: cavium: fix NULL pointer dereference in cavium_ptp_put

compiler: gcc (GCC) 7.1.1 20170620
Patch is attached.
Kernel config is attached.


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