net: sctp: fix smatch warning in sctp_send_asconf_del_ip

This was originally reported in [1] and posted by Neil Horman [2], he said:

  Fix up a missed null pointer check in the asconf code. If we don't find
  a local address, but we pass in an address length of more than 1, we may
  dereference a NULL laddr pointer. Currently this can't happen, as the only
  users of the function pass in the value 1 as the addrcnt parameter, but
  its not hot path, and it doesn't hurt to check for NULL should that ever
  be the case.

The callpath from sctp_asconf_mgmt() looks okay. But this could be triggered
from sctp_setsockopt_bindx() call with SCTP_BINDX_REM_ADDR and addrcnt > 1
while passing all possible addresses from the bind list to SCTP_BINDX_REM_ADDR
so that we do *not* find a single address in the association's bind address
list that is not in the packed array of addresses. If this happens when we
have an established association with ASCONF-capable peers, then we could get
a NULL pointer dereference as we only check for laddr == NULL && addrcnt == 1
and call later sctp_make_asconf_update_ip() with NULL laddr.

BUT: this actually won't happen as sctp_bindx_rem() will catch such a case
and return with an error earlier. As this is incredably unintuitive and error
prone, add a check to catch at least future bugs here. As Neil says, its not
hot path. Introduced by 8a07eb0a ("sctp: Add ASCONF operation on the
single-homed host").

 [1] http://www.spinics.net/lists/linux-sctp/msg02132.html
 [2] http://www.spinics.net/lists/linux-sctp/msg02133.html



Reported-by: Dan Carpenter <dan.carpenter@oracle.com>
Signed-off-by: Neil Horman <nhorman@tuxdriver.com>
Signed-off-by: Daniel Borkmann <dborkman@redhat.com>
Cc: Michio Honda <micchie@sfc.wide.ad.jp>
Acked-By: Neil Horman <nhorman@tuxdriver.com>
Acked-by: Vlad Yasevich <vyasevich@gmail.com>
Signed-off-by: David S. Miller <davem@davemloft.net>