Shimcachemem: Changes absent value return type

This is more appropriately set to `renderers.UnreadableValue` since it
is set when an `exceptions.InvalidAddressException` occurs.

Author: dgmcdona

volatility3/framework/symbols/windows/extensions/shimcache.py

@@ -57,7 +57,7 @@ def exec_flag(self) -> Union[bool, interfaces.renderers.BaseAbsentValue]:
                 if not self._context.layers[self.vol.native_layer_name].is_valid(
                     blob_offset, blob_size
                 ):
-                    self._exec_flag = renderers.UnparsableValue()
+                    self._exec_flag = renderers.UnreadableValue()
                     return self._exec_flag
 
                 raw_flag = self._context.layers[self.vol.native_layer_name].read(