Fix crash when expanding variables in post-mortem

[qhuy4119]

Currently, Debugger.stack will be empty when entering post-mortem, leading to IndexError when user tries to expand variables in the Variables window. This commit fixes that by restoring Debugger.bottom_frame to Debugger.stack.

fixes #233

[inducer]

This actually isn't right. Post-mortem mode also kicks in if the debuggee runs into an exception. Your proposed change appears to break inspecting the stack in that situation.