a defensive programming measure.
Note that whilst these members are not used by the ip_output()
path, we are passing an instance of struct ip_moptions here
which is declared on the stack (which could be considered a
bad thing).
ip_output() does not consume struct ip_moptions, but in case it
does in future, declare an in_multi vector on the stack too to
behave more like ip_findmoptions() does.
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