[SCTP]: Return socket errors only if the receive queue is empty.

This patch fixes an issue where it is possible to get valid data after
a ENOTCONN error. It returns socket errors only after data queued on
socket receive queue is consumed.

Signed-off-by: Neil Horman <nhorman@tuxdriver.com>
Signed-off-by: Sridhar Samudrala <sri@us.ibm.com>
Signed-off-by: David S. Miller <davem@davemloft.net>