[SCTP]: Fix build warning in sctp_sf_do_5_1C_ack().

Reported by Andrew Morton.

net/sctp/sm_statefuns.c: In function 'sctp_sf_do_5_1C_ack':
net/sctp/sm_statefuns.c:484: warning: 'error' may be used uninitialized in this function

Signed-off-by: David S. Miller <davem@davemloft.net>