mfreadwrite: Block recurring flush calls in async mode when flush is pending.

dlls/mfreadwrite/reader.c

@@ -1710,6 +1710,9 @@ static HRESULT source_reader_flush_async(struct source_reader *reader, unsigned
     unsigned int stream_index;
     HRESULT hr;
 
+    if (reader->flags & SOURCE_READER_FLUSHING)
+        return MF_E_INVALIDREQUEST;
+
     switch (index)
     {
         case MF_SOURCE_READER_FIRST_VIDEO_STREAM:

dlls/mfreadwrite/tests/mfplat.c

@@ -951,6 +951,13 @@ todo_wine
     hr = IMFSourceReader_ReadSample(reader, 0, 0, NULL, NULL, NULL, &sample);
     ok(hr == E_INVALIDARG, "Unexpected hr %#x.\n", hr);
 
+    /* Flush() arguments validation. */
+    hr = IMFSourceReader_Flush(reader, 123);
+    ok(hr == MF_E_INVALIDSTREAMNUMBER, "Unexpected hr %#x.\n", hr);
+
+    hr = IMFSourceReader_ReadSample(reader, 0, 0, NULL, NULL, NULL, NULL);
+    ok(hr == MF_E_NOTACCEPTING, "Unexpected hr %#x.\n", hr);
+
     IMFSourceReader_Release(reader);
 }