Objects: Fix incorrect Bitfield maths

Turns out that #133 exposed a mistake in how we were using the end_bit
field (and it should have been picked up in review, my bad).
Essentially we were masking based on the length of the end_bit after
*already* shifting by the start_bit.  We should mask then shift, not the
other way around.

May well fix issue #135 (pdb generation will have been affected by
this).

volatility/framework/objects/__init__.py

@@ -367,7 +367,7 @@ def __new__(cls,
                 end_bit: int = 0,
                 **kwargs) -> 'BitField':
         value = base_type(context = context, object_info = object_info)
-        return int.__new__(cls, (value >> start_bit) & ((1 << end_bit) - 1))  # type: ignore
+        return int.__new__(cls, ((value & ((1 << end_bit) - 1)) >> start_bit))  # type: ignore
 
     def write(self, value):
         raise NotImplementedError("Writing to BitFields is not yet implemented")