dpif-linux: Recheck the socket pointer existence before getting its pid.

This commit fixes a race between port deletion and flow miss handling.
More specifically, a port could be removed by main thread while
the handler thread is handling the flow miss from it.  If the flow
requires slow path action, the handler thread will try querying a pid
from port's socket.  Since the port has been deleted, the query will
cause a dereference of NULL socket pointer.

This commit makes the handler thread recheck the socket pointer before
dereferencing it.

VMware-BZ: 1251981

Reported-by: Pratap Reddy <preddy@nicira.com>
Signed-off-by: Alex Wang <alexw@nicira.com>
Acked-by: Joe Stringer <joestringer@nicira.com>