Fix the truncation of strings

MHL kindly pointed out that the string wasn't truncating, and it turns
out I misread the struct documentation, which truncates or pads with
\x00s to ensure the appropriate length.  We now truncate to the first
null we find.

volatility/framework/objects/__init__.py

@@ -140,7 +140,8 @@ class String(PrimitiveObject, str):
                                                            layer_name = object_info.layer_name,
                                                            offset = object_info.offset),
                                          **params)
-        # We don't truncate on "\x00" because the string decoder does that for us
+        if value.find('\x00') >= 0:
+            value = value[:value.find('\x00')]
         return value