Fix qlinear_viewer to avoid error on wheelEvent

[xxr0ss]

When switch Disassembly view to the linear view and scroll the view while the binary is still loading (the view is still blank), an TypeError will raise

  File "/xxx/angrmanagement/ui/widgets/qlinear_viewer.py", line 169, in wheelEvent
    self.verticalScrollBar().setValue(self.offset * self._line_height)
                                      ~~~~~~~~~~~~^~~~~~~~~~~~~~~~~~~
TypeError: unsupported operand type(s) for *: 'NoneType' and 'float'

Cause:

self.offset could still be None after prepare_objects() and when wheelEvent() uses it to do calculations, error raises