Thread worker: Error sending packet, when num_threads > 2

This is my test code
```
vr = decord.VideoReader('input\\videoS.webm', ctx=decord.cpu(0), num_threads=2)
for i in range(len(vr)):
frame = vr[i]
crop = frame.asnumpy()[500:550, 800:850]
```
When I put `num_threads` higher then 2 it throws a error, but only for some video files.
```
Traceback (most recent call last):
File "c:\Users\GG\Desktop\LoL-OCR\test.py", line 12, in
frame = vr[i]
File "C:\Users\GG\.conda\envs\LoL\lib\site-packages\decord\video_reader.py", line 105, in __getitem__
return self.next()
File "C:\Users\GG\.conda\envs\LoL\lib\site-packages\decord\video_reader.py", line 117, in next
arr = _CAPI_VideoReaderNextFrame(self._handle)
File "C:\Users\GG\.conda\envs\LoL\lib\site-packages\decord\_ffi\_ctypes\function.py", line 173, in __call__
check_call(_LIB.DECORDFuncCall(
File "C:\Users\GG\.conda\envs\LoL\lib\site-packages\decord\_ffi\base.py", line 78, in check_call
raise DECORDError(err_str)
decord._ffi.base.DECORDError: [22:49:58] D:\a\decord\decord\src\video\ffmpeg\threaded_decoder.cc:292: [22:49:58] D:\a\decord\decord\src\video\ffmpeg\threaded_decoder.cc:218: Check failed: avcodec_send_packet(dec_ctx_.get(), pkt.get()) >= 0 (-11 vs. 0) Thread worker: Error sending packet.
```
And if I remove `crop = frame.asnumpy()[500:550, 800:850]` when `num_threads` is higher then 2 it throws this error and again only for some video files.
```
Traceback (most recent call last):
File "c:\Users\GG\Desktop\LoL-OCR\test2.py", line 7, in
frame = vr[i]
File "C:\Users\GG\.conda\envs\LoL\lib\site-packages\decord\video_reader.py", line 105, in __getitem__
return self.next()
File "C:\Users\GG\.conda\envs\LoL\lib\site-packages\decord\video_reader.py", line 117, in next
arr = _CAPI_VideoReaderNextFrame(self._handle)
File "C:\Users\GG\.conda\envs\LoL\lib\site-packages\decord\_ffi\_ctypes\function.py", line 173, in __call__
check_call(_LIB.DECORDFuncCall(
File "C:\Users\GG\.conda\envs\LoL\lib\site-packages\decord\_ffi\base.py", line 78, in check_call
raise DECORDError(err_str)
decord._ffi.base.DECORDError: [22:57:11] D:\a\decord\decord\src\video\ffmpeg\threaded_decoder.cc:104: Check failed: run_.load()
```

* OS: Windows 10
* Conda: 22.9.0
* Python: 3.10.8
* Pip: 22.3.1
* Decord: 0.6.0
* Ffmpeg: 4.2.3