Error with mxnet 1.8.0 dataloader

With mxnet 1.8.0.post0, importing decord _before_ mxnet causes an error using a dataloader.

```python
import decord
import mxnet
from mxnet import gluon, image
from mxnet.gluon.data import DataLoader

class ImageDataset(gluon.data.Dataset):
def __init__(self, paths):
self._all_paths = paths

def __len__(self):
return len(self._all_paths)

def __getitem__(self, item):
image_path = self._all_paths[item]
img = image.imread(image_path, to_rgb=True)
return img

def get_dataloader(self, **kwargs):
return DataLoader(self, **kwargs)

print("decord", decord.__version__)
print("mxnet", mxnet.__version__)
paths = ['0.png', '1.png']
loader = ImageDataset(paths).get_dataloader(batch_size=2)
a, *_ = loader
print(a.shape, a.asnumpy().mean())
```

That snippet (the files obviously exist and are valid images) yields:
```
decord 0.5.2
mxnet 1.8.0
libpng error: bad parameters to zlib
libpng error: bad parameters to zlib
Traceback (most recent call last):
File "repro.py", line 27, in
print(a.shape, a.asnumpy().mean())
File "/home/leonardo/miniconda3/envs/mx2/lib/python3.7/site-packages/mxnet/ndarray/ndarray.py", line 2566, in asnumpy
ctypes.c_size_t(data.size)))
File "/home/leonardo/miniconda3/envs/mx2/lib/python3.7/site-packages/mxnet/base.py", line 246, in check_call
raise get_last_ffi_error()
mxnet.base.MXNetError: Traceback (most recent call last):
File "../src/io/image_io.cc", line 160
MXNetError: Check failed: !dst.empty(): Decoding failed. Invalid image file.
[bt] (0) /home/leonardo/miniconda3/envs/mx2/lib/python3.7/site-packages/decord/libdecord.so(dmlc::StackTrace(unsigned long)+0x85) [0x7faeb7f3962c]
[bt] (1) /home/leonardo/miniconda3/envs/mx2/lib/python3.7/site-packages/decord/libdecord.so(dmlc::LogMessageFatal::~LogMessageFatal()+0x36) [0x7faeb7f3991c]
[bt] (2) /home/leonardo/miniconda3/envs/mx2/lib/python3.7/site-packages/mxnet/libmxnet.so(mxnet::io::ImdecodeImpl(int, bool, void*, unsigned long, mxnet::NDArray*)+0x521) [0x7faeabb5b7a1]
[bt] (3) /home/leonardo/miniconda3/envs/mx2/lib/python3.7/site-packages/mxnet/libmxnet.so(+0x122296e) [0x7faeaba9f96e]
[bt] (4) /home/leonardo/miniconda3/envs/mx2/lib/python3.7/site-packages/mxnet/libmxnet.so(mxnet::engine::ThreadedEngine::ExecuteOprBlock(mxnet::RunContext, mxnet::engine::OprBlock*)+0x44c) [0x7faeabaa9dbc]
[bt] (5) /home/leonardo/miniconda3/envs/mx2/lib/python3.7/site-packages/mxnet/libmxnet.so(std::_Function_handler), mxnet::engine::ThreadedEnginePerDevice::PushToExecute(mxnet::engine::OprBlock*, bool)::{lambda()#1}::operator()() const::{lambda(std::shared_ptr)#1}>::_M_invoke(std::_Any_data const&, std::shared_ptr)+0xc4) [0x7faeabaabab4]
[bt] (6) /home/leonardo/miniconda3/envs/mx2/lib/python3.7/site-packages/mxnet/libmxnet.so(std::thread::_Impl)> (std::shared_ptr)> >::_M_run()+0x3b) [0x7faeabaa8eab]
[bt] (7) /home/leonardo/miniconda3/envs/mx2/lib/python3.7/site-packages/decord/libdecord.so(+0x166610) [0x7faeb7fa8610]
[bt] (8) /lib/x86_64-linux-gnu/libpthread.so.0(+0x76db) [0x7faed03ea6db]
[bt] (9) /lib/x86_64-linux-gnu/libc.so.6(clone+0x3f) [0x7faed011371f]

Error in atexit._run_exitfuncs:
Traceback (most recent call last):
File "/home/leonardo/miniconda3/envs/mx2/lib/python3.7/site-packages/mxnet/base.py", line 587, in _notify_shutdown
check_call(_LIB.MXNotifyShutdown())
File "/home/leonardo/miniconda3/envs/mx2/lib/python3.7/site-packages/mxnet/base.py", line 246, in check_call
raise get_last_ffi_error()
mxnet.base.MXNetError: Traceback (most recent call last):
File "../src/io/image_io.cc", line 160
MXNetError: Check failed: !dst.empty(): Decoding failed. Invalid image file.
[bt] (0) /home/leonardo/miniconda3/envs/mx2/lib/python3.7/site-packages/decord/libdecord.so(dmlc::StackTrace(unsigned long)+0x85) [0x7faeb7f3962c]
[bt] (1) /home/leonardo/miniconda3/envs/mx2/lib/python3.7/site-packages/decord/libdecord.so(dmlc::LogMessageFatal::~LogMessageFatal()+0x36) [0x7faeb7f3991c]
[bt] (2) /home/leonardo/miniconda3/envs/mx2/lib/python3.7/site-packages/mxnet/libmxnet.so(mxnet::io::ImdecodeImpl(int, bool, void*, unsigned long, mxnet::NDArray*)+0x521) [0x7faeabb5b7a1]
[bt] (3) /home/leonardo/miniconda3/envs/mx2/lib/python3.7/site-packages/mxnet/libmxnet.so(+0x122296e) [0x7faeaba9f96e]
[bt] (4) /home/leonardo/miniconda3/envs/mx2/lib/python3.7/site-packages/mxnet/libmxnet.so(mxnet::engine::ThreadedEngine::ExecuteOprBlock(mxnet::RunContext, mxnet::engine::OprBlock*)+0x44c) [0x7faeabaa9dbc]
[bt] (5) /home/leonardo/miniconda3/envs/mx2/lib/python3.7/site-packages/mxnet/libmxnet.so(std::_Function_handler), mxnet::engine::ThreadedEnginePerDevice::PushToExecute(mxnet::engine::OprBlock*, bool)::{lambda()#1}::operator()() const::{lambda(std::shared_ptr)#1}>::_M_invoke(std::_Any_data const&, std::shared_ptr)+0xc4) [0x7faeabaabab4]
[bt] (6) /home/leonardo/miniconda3/envs/mx2/lib/python3.7/site-packages/mxnet/libmxnet.so(std::thread::_Impl)> (std::shared_ptr)> >::_M_run()+0x3b) [0x7faeabaa8eab]
[bt] (7) /home/leonardo/miniconda3/envs/mx2/lib/python3.7/site-packages/decord/libdecord.so(+0x166610) [0x7faeb7fa8610]
[bt] (8) /lib/x86_64-linux-gnu/libpthread.so.0(+0x76db) [0x7faed03ea6db]
[bt] (9) /lib/x86_64-linux-gnu/libc.so.6(clone+0x3f) [0x7faed011371f]
```
Swap the imports:
```python
import mxnet
from mxnet import gluon, image
from mxnet.gluon.data import DataLoader
import decord

...
```

and everything works as expected (see below). By the way, with previous versions (I tested mxnet 1.5.1) everything works in both cases (as it should). Different versions of decord don't really seem to matter.
```
decord 0.5.2
mxnet 1.8.0
(2, 720, 1280, 3) 117.8157691333912
```