RFR 8191025: (ch) Scattering reads to a subsequence of buffers ignores length

[Brian Burkhalter]

https://bugs.openjdk.java.net/browse/JDK-8191025
http://cr.openjdk.java.net/~bpb/8191025/webrev.00/

bufs.length was inadvertently passed instead of length resulting in a NPE when length < bufs.length and bufs[idx] == null for some idx in [length,bufs.length).

Thanks,

Brian