stream: remove unreachable code

First commit remove some unreachable code. BufferList.prototype.concat() is not called when there is only a buffer in the list.

Second commit fixes a test.

Checklist

• make -j4 test (UNIX), or vcbuild test (Windows) passes
• tests and/or benchmarks are included
• documentation is changed or added
• commit message follows commit guidelines

Affected core subsystem(s)

stream, test