pim6d: Fixing core while running MLD conformance test.

pim6d: Fixing core while running MLD conformance test.

While running MLD conformance test 9.2 core is getting generated.

Test setps:
1. ANVL: Listen (for upto <GeneralQueryRecvWaitTime> seconds) on <AIface-0>.
2. DUT: Send MLD General Query Message.
3. ANVL: Send MLD Report Message to <DIface-0> containing:
• IPv6 Source Address field set to link-local IPv6 Address of HOST-1
• IPv6 Destination Address field set to <McastAddrGroup>
• MLD Multicast Address field set to <McastAddrGroup>.
4. ANVL: Wait for <ProcessTime> seconds for DUT to process and add <Mcas- tAddrGroup> to its Multicast Address list.
5. ANVL: Send MLD General Query Message to <DIface-0> containing:
• IPv6 Source Address field set to link-local IPv6 Address of RTR-1 which is numerically less than the link-local IPv6 unicast address of <DIface-0>
• IPv6 Destination Address field set to link-scope all-nodes multicast address.
6. ANVL: Send MLD Multicast-Address-Specific Query Message to <DIface-0> containing:
• IPv6 Source Address field set to link-local IPv6 Address of RTR-1
• IPv6 Destination Address field set to <McastAddrGroup>
• MLD Multicast Address field set to <McastAddrGroup>
• MLD Maximum Response Delay field value set to 0.
7. ANVL: Verify that the Maximum Response Delay timer for <McastAd- drGroup> is set to zero.

While running above test, when group specific query is received we start gm_t_sg_expire timer.
Once this timer expires, we clear the corresponding entry.
During this sg->state was still set to JOIN. This happened because receiver went down without sending leave.

Added a condition to update the sg->state before starting the timer.
If receiver goes down without sending leave we will update sg->state to GM_SG_JOIN_EXPIRING or GM_SG_NOPRUNE_EXPIRING based on previous state.
If we receive a join then sg->state will be refreshed and will be updated to JOIN state.

Fixes: #13387
Signed-off-by: Abhishek N R <abnr@vmware.com>