6984545: (fc) transferFrom does not throw NonReadableChannelException when target is size 0 and non-readable

[Alan Bateman]

The changes for 6431344 introduced a small regression. If FileChannel's 
transferFrom is invoked with a source that is non-readable FileChannel 
then NonReadableChannelException is not thrown when there are no bytes 
to transfer. The fix is to trivially check if the source is readable:
    http://cr.openjdk.java.net/~alanb/6984545/webrev/

Thanks,
Alan.

[1] http://hg.openjdk.java.net/jdk7/jdk7/jdk/rev/f4a2b4e7a831