uninitialized source address for source specific multicast

[cgang]

There is an issue with source specific multicast (SSM) support, that source address is uninitialized if SSM unspecified.
Which may cause mcast listen with wrong parameter since set_multicast() rely on following condition (netop.c, line 158):

if (s_in_addr->s_addr != 0)

It will result in socket timeout if source address is unspecified and uninitialized.