pool: Fix NPE in HTTP mover

When a transfer doing a chunked upload is killed, the following
NPE is caught when several chunks are in the queue already.

10 apr 2013 12:28:50 (pool_write) [] Uncaught exception in thread http-disk-83
java.lang.NullPointerException: null
        at java.util.concurrent.ConcurrentHashMap.hash(ConcurrentHashMap.java:332) ~[na:1.7.0_11]
        at java.util.concurrent.ConcurrentHashMap.get(ConcurrentHashMap.java:987) ~[na:1.7.0_11]
        at org.dcache.pool.movers.AbstractNettyServer.close(AbstractNettyServer.java:235) ~[dcache-core-2.6.0-SNAPSHOT.jar:2.6.0-SNAPSHOT]
        at org.dcache.http.HttpPoolRequestHandler.close(HttpPoolRequestHandler.java:620) ~[dcache-core-2.6.0-SNAPSHOT.jar:2.6.0-SNAPSHOT]
        at org.dcache.http.HttpPoolRequestHandler.doOnChunk(HttpPoolRequestHandler.java:551) ~[dcache-core-2.6.0-SNAPSHOT.jar:2.6.0-SNAPSH
OT]
        at org.dcache.http.HttpRequestHandler.messageReceived(HttpRequestHandler.java:63) ~[dcache-core-2.6.0-SNAPSHOT.jar:2.6.0-SNAPSHOT]
        at org.jboss.netty.handler.timeout.IdleStateAwareChannelHandler.handleUpstream(IdleStateAwareChannelHandler.java:48) ~[netty-3.2.7
.Final.jar:na]
        at org.jboss.netty.handler.stream.ChunkedWriteHandler.handleUpstream(ChunkedWriteHandler.java:149) ~[netty-3.2.7.Final.jar:na]
        at org.jboss.netty.handler.timeout.IdleStateHandler.messageReceived(IdleStateHandler.java:275) ~[netty-3.2.7.Final.jar:na]
        at org.jboss.netty.handler.execution.ChannelEventRunnable.run(ChannelEventRunnable.java:69) [netty-3.2.7.Final.jar:na]
        at org.jboss.netty.handler.execution.OrderedMemoryAwareThreadPoolExecutor$ChildExecutor.run(OrderedMemoryAwareThreadPoolExecutor.j
ava:316) [netty-3.2.7.Final.jar:na]
        at java.util.concurrent.ThreadPoolExecutor.runWorker(ThreadPoolExecutor.java:1110) [na:1.7.0_11]
        at java.util.concurrent.ThreadPoolExecutor$Worker.run(ThreadPoolExecutor.java:603) [na:1.7.0_11]
        at org.dcache.util.CDCThreadFactory$1.run(CDCThreadFactory.java:42) [dcache-core-2.6.0-SNAPSHOT.jar:2.6.0-SNAPSHOT]
        at java.lang.Thread.run(Thread.java:722) [na:1.7.0_11]

The problem is caused by falsely closing the mover several times. The
error message generated is also misleading.

Target: trunk
Require-notes: no
Require-book: no
Request: 2.2
Request: 2.5
Acked-by: Tigran Mkrtchyan <tigran.mkrtchyan@desy.de>
Patch: http://rb.dcache.org/r/5443/
Signed-off-by: Tigran Mkrtchyan <tigran.mkrtchyan@desy.de>