DeviceMemoryPool causes crash at shutdown

Consider the following piece of code:

```
import numpy as np
import pycuda.autoinit
import pycuda.gpuarray as gpu
from pycuda.tools import DeviceMemoryPool

cuda_memory_pool = DeviceMemoryPool()
X = np.array([1, 2, 3.0])
X = gpu.to_gpu(X, allocator=cuda_memory_pool.allocate)
```

On my machine, this gives:

```
python2.7 test.py
terminate called after throwing an instance of 'pycuda::error'
what(): explicit_context_dependent failed: invalid context - no currently active context?
Aborted
```

This crash happens AFTER all cleanup operations, somewhere in the guts of pycuda. (At this stage, the `_finish_up()` routine from pycuda.autoinit has already been run). While the main script runs through, this makes it impossible to profile scripts through the Nvidia Visual Profiler, which is rather annoying.