ipv4: igmp: Remove redundant comparison in igmp_mcf_get_next()

The 'state->im' value will always be non-zero after
the 'while' statement, so the check can be removed.

Found by InfoTeCS on behalf of Linux Verification Center
(linuxtesting.org) with SVACE.

Signed-off-by: Gavrilov Ilia <Ilia.Gavrilov@infotecs.ru>
Reviewed-by: David Ahern <dsahern@kernel.org>
Link: https://lore.kernel.org/r/20230912084039.1501984-1-Ilia.Gavrilov@infotecs.ru
Signed-off-by: Paolo Abeni <pabeni@redhat.com>