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.

tests/zstreamtest.c

@@ -1695,8 +1695,9 @@ static int fuzzerTests_newAPI(U32 seed, U32 nbTests, unsigned startTest, double
                     size_t const randomDstSize = FUZ_randomLength(&lseed, maxSampleLog+1);
                     size_t const adjustedDstSize = MIN(cBufferSize - cSize, randomDstSize);
                     outBuff.size = outBuff.pos + adjustedDstSize;
-                    DISPLAYLEVEL(6, "End-flush into dst buffer of size %u \n", (U32)adjustedDstSize);
+                    DISPLAYLEVEL(6, "t%u: End-flush into dst buffer of size %u \n", testNb, (U32)adjustedDstSize);
                     remainingToFlush = ZSTD_compress_generic(zc, &outBuff, &inBuff, ZSTD_e_end);
+                    DISPLAYLEVEL(6, "t%u: Total flushed so far : %u bytes \n", testNb, (U32)outBuff.pos);
                     CHECK( ZSTD_isError(remainingToFlush),
                           "ZSTD_compress_generic w/ ZSTD_e_end error : %s",
                            ZSTD_getErrorName(remainingToFlush) );