pimd: return check (Coverity 1465490)

pimd: return check (Coverity 1465490)

Unless someone intentionally changes MCAST_ALL_ROUTERS ("224.0.0.2") with a
wrong IP, this should never fail, so the fix is using "(void)" at the left
of the function call, as an explicit way of indicating we discard the
return value on purpose.

Signed-off-by: F. Aragon <paco@voltanet.io>