Edited Issue: [0.3.x] isValid rasies MemoryException with typedVar [13744]

```
>>> isValid(0x0) # returning False is expected
False
>>> isValid(typedVar("UInt4B", 0)) # raising MemoryException is not expected
Traceback (most recent call last):
File "<stdin>", line 1, in <module>
__main__.MemoryException: Memory exception at 0x0 target virtual address
```