Reading nested NoopStreams with sharebuf=false produces garbage data

[nhz2]

Here is an example:

using Test
using TranscodingStreams
stream = NoopStream(NoopStream(IOBuffer("foobar")); sharedbuf=false)
@test map(x->read(stream, UInt8), 1:6) == b"foobar"

Gives random bytes:

 Evaluated: UInt8[0x70, 0x2c, 0x93, 0x02, 0x00, 0x00] == UInt8[0x66, 0x6f, 0x6f, 0x62, 0x61, 0x72]

This bug was added in #168