zstdmt : fix end condition (ZSTD_e_end)

When ZSTD_e_end directive is provided,
the question is not only "are internal buffers completely flushed",
it is also "is current frame completed".

In some rare cases,
it was possible for internal buffers to be completely flushed,
triggering a @return == 0,
but frame was not completed as it needed a last null-size block to mark the end,
resulting in an unfinished frame.

lib/zstd.h

@@ -1136,10 +1136,11 @@ typedef enum {
  *                                                     and then immediately returns, just indicating that there is some data remaining to be flushed.
  *                                                     The function nonetheless guarantees forward progress : it will return only after it reads or write at least 1+ byte.
  *  - Exception : in multi-threading mode, if the first call requests a ZSTD_e_end directive, it is blocking : it will complete compression before giving back control to caller.
- *  - @return provides the minimum amount of data remaining to be flushed from internal buffers
+ *  - @return provides a minimum amount of data remaining to be flushed from internal buffers
  *            or an error code, which can be tested using ZSTD_isError().
  *            if @return != 0, flush is not fully completed, there is still some data left within internal buffers.
- *            This is useful to determine if a ZSTD_e_flush or ZSTD_e_end directive is completed.
+ *            This is useful for ZSTD_e_flush, since in this case more flushes are necessary to empty all buffers.
+ *            For ZSTD_e_end, @return == 0 when internal buffers are fully flushed and frame is completed.
  *  - after a ZSTD_e_end directive, if internal buffer is not fully flushed (@return != 0),
  *            only ZSTD_e_end or ZSTD_e_flush operations are allowed.
  *            Before starting a new compression job, or changing compression parameters,