igmp: fix igmp proxy group merge

When merging proxy groups in igmp_proxy_device_merge_group(), the call
to igmp_proxy_device_merge_src() can end up removing the current proxy
group via igmp_group_clear(). When that happens, it must returns NULL so
that igmp_proxy_device_merge_config() does not send a IGMPv3 report for
a dead proxy group.
Make igmp_group_clear() reset the group pointer to NULL to fix this bug
and to detect similar bugs more easily.

Type: fix

Change-Id: I229e55b5bfa71734d7844893f5209a66fa3cc8ae
Signed-off-by: Benoît Ganne <bganne@cisco.com>