incorrect use of buffer pool

[marten-seemann]

In Read, we reslice the buffer:

go-yamux/util.go

Line 144 in c8e9b3e

s.b[0] = s.b[0][n:]

When returning this buffer to the pool, it will have a shorter length than the buffer we took from the buffer. Even worse, as we're using a pool, the bytes at the beginning of the buffer will be wasted eternally.