Fix limit computation of NIO ByteBuffers obtained via ReadOnlyByteBufferBuf.nioBuffer

Motivation:

When starting with a read-only NIO buffer, wrapping it in a ByteBuf,
and then later retrieving a re-wrapped NIO buffer the limit was getting
too short.

Modifications:

Changed ReadOnlyByteBufferBuf.nioBuffer(int,int) to compute the
limit in the same manner as the internalNioBuffer method.

Result:

Round-trip conversion from NIO to ByteBuf to NIO will work reliably.