[lib] Avoid allocating the input buffer when ZSTD_c_stableInBuffer is set

[lib] Avoid allocating the input buffer when ZSTD_c_stableInBuffer is set

We don't use it when we have a stable input buffer, so don't allocate
it. I had to slightly modify `ZSTD_copyCCtx()` by storing the
`ZSTD_buffered_policy_e` in the `ZSTD_CCtx`, since `inBuffSize > 0` is
no longer the correct signal for the buffered mode.