pimd: Cleanup uninitialized memory access

pimd: Cleanup uninitialized memory access

Valgrind noticed that we have a read of uninitialized memory:

Conditional jump or move depends on uninitialised value(s)
==13749==    at 0x428067: pim_ifassert_winner_set (pim_assert.c:57)
==13749==    by 0x4266F0: pim_ifchannel_add (pim_ifchannel.c:535)
==13749==    by 0x426CC1: pim_ifchannel_join_add (pim_ifchannel.c:730)
==13749==    by 0x427B5B: recv_join (pim_join.c:95)
==13749==    by 0x427B5B: pim_joinprune_recv (pim_join.c:270)
==13749==    by 0x42354F: pim_pim_packet (pim_pim.c:249)
==13749==    by 0x4236C0: pim_sock_read (pim_pim.c:349)
==13749==    by 0x4E60587: thread_call (thread.c:1462)
==13749==    by 0x40C75E: main (pim_main.c:266)
==13749==

This commit fixes that issue.

Signed-off-by: Donald Sharp <sharpd@cumulusnetworks.com>