8219597: (bf) Heap buffer state changes could provoke unexpected exceptions

[Brian Burkhalter]

This issue [1] was instigated by this comment [2]. The change [3] hopefully reduces the exposure of heap buffers to changes in the buffer state, notably the position (cursor).

Thanks,

Brian

[1] https://bugs.openjdk.java.net/browse/JDK-8219597
[2] http://mail.openjdk.java.net/pipermail/nio-dev/2019-February/005900.html
[3] http://cr.openjdk.java.net/~bpb/8219597/webrev.00/