cli: fixed wrong initialization in MT mode

cli: fixed wrong initialization in MT mode

It's not good to mix old and new API
ZSTD_resetCStream() doesn't just set pledgedSrcSize :
it also sets the CCtx for a single thread compression.

Problem is, when 2+ threads are defined in cctx->requestedParams,
ZSTD_compress_generic() will want to start MT compression,
since initialization is supposed to have already happened (thanks to ZSTD_resetCStream())
except that the underlying ZSTDMT_CCtx* object is not created,
resulting in a segfault.

This is an invalid construction
(correct one is to use ZSTD_CCtx_setPledgedSrcSize()).
I haven't found a nice way to mitigate this impact if someone makes the same mistake.

At some point, removing the old API to keep only the new API within fileio.c will limit these risks.