rds: Fix warning.

>> net/rds/send.c:1109:42: warning: Using plain integer as NULL pointer

Fixes: ea010070 ("net/rds: fix warn in rds_message_alloc_sgs")
Reported-by: kbuild test robot <lkp@intel.com>
Signed-off-by: David S. Miller <davem@davemloft.net>