Deal with the case where ifma_protospec may be NULL, during

Deal with the case where ifma_protospec may be NULL, during
any IPv4 multicast operations which reference it.

There is a potential race because ifma_protospec is set to NULL
when we discover the underlying ifnet has gone away. This write
is not covered by the IF_ADDR_LOCK, and it's difficult to widen
its scope without making it a recursive lock. It isn't clear why
this manifests more quickly with 802.11 interfaces, but does not
seem to manifest at all with wired interfaces.

With this change, the 802.11 related panics reported by sam@
and cokane@ should go away. It is not the right fix, that requires
more thought before 8.0.

Idea from:	sam
Tested by:	cokane