[lib] Take the shortcut when ZSTD_c_stableOutBuffer is set
[lib] Take the shortcut when ZSTD_c_stableOutBuffer is set

When we have a stable output buffer take the single-pass shortcut.
It is okay to return `dstSizeTooSmall` if the output buffer isn't
big enough, because we know it will never grow.

Cyan4973/zstd,Cyan4973/zstd,Cyan4973/zstd,Cyan4973/zstd,Cyan4973/zstd