ipv4: Remove erroneous check in igmpv3_newpack() and igmp_send_report().

Output route resolution never returns a route with rt_src set to zero
(which is INADDR_ANY).

Even if the flow key for the output route lookup specifies INADDR_ANY
for the source address, the output route resolution chooses a real
source address to use in the final route.

This test has existed forever in igmp_send_report() and David Stevens
simply copied over the erroneous test when implementing support for
IGMPv3.

Signed-off-by: David S. Miller <davem@davemloft.net>
Reviewed-by: Eric Dumazet <eric.dumazet@gmail.com>