check endDirective in ZSTD_compressStream2()

fix #2297
also :
- `assert()` `endDirective` in `ZSTD_compressStream_internal()`, for debug mode
- add relevant tests

lib/compress/zstd_compress.c

@@ -3937,6 +3937,7 @@ static size_t ZSTD_compressStream_generic(ZSTD_CStream* zcs,
     assert(zcs->outBuffSize > 0);
     assert(output->pos <= output->size);
     assert(input->pos <= input->size);
+    assert((U32)flushMode <= (U32)ZSTD_e_end);
 
     while (someMoreWork) {
         switch(zcs->streamStage)
@@ -4083,9 +4084,10 @@ size_t ZSTD_compressStream2( ZSTD_CCtx* cctx,
 {
     DEBUGLOG(5, "ZSTD_compressStream2, endOp=%u ", (unsigned)endOp);
     /* check conditions */
-    RETURN_ERROR_IF(output->pos > output->size, GENERIC, "invalid buffer");
-    RETURN_ERROR_IF(input->pos  > input->size, GENERIC, "invalid buffer");
-    assert(cctx!=NULL);
+    RETURN_ERROR_IF(output->pos > output->size, dstSize_tooSmall, "invalid output buffer");
+    RETURN_ERROR_IF(input->pos  > input->size, srcSize_wrong, "invalid input buffer");
+    RETURN_ERROR_IF((U32)endOp > (U32)ZSTD_e_end, parameter_outOfBound, "invalid endDirective");
+    assert(cctx != NULL);
 
     /* transparent initialization stage */
     if (cctx->streamStage == zcss_init) {

lib/zstd.h

@@ -677,8 +677,9 @@ typedef enum {
  *  - Compression parameters cannot be changed once compression is started (save a list of exceptions in multi-threading mode)
  *  - output->pos must be <= dstCapacity, input->pos must be <= srcSize
  *  - output->pos and input->pos will be updated. They are guaranteed to remain below their respective limit.
+ *  - endOp must be a valid directive
  *  - When nbWorkers==0 (default), function is blocking : it completes its job before returning to caller.
- *  - When nbWorkers>=1, function is non-blocking : it just acquires a copy of input, and distributes jobs to internal worker threads, flush whatever is available,
+ *  - When nbWorkers>=1, function is non-blocking : it copies a portion of input, distributes jobs to internal worker threads, flush to output whatever is available,
  *                                                  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 : if the first call requests a ZSTD_e_end directive and provides enough dstCapacity, the function delegates to ZSTD_compress2() which is always blocking.