Add stack and local_vars to ZStackBottom sentinel

ZStackBottom lacked attributes that normal stack operations need when
returning from a subroutine to the top level. Zork 1 hit this on the
first subroutine return during startup.

src/mudlib/zmachine/zstackmanager.py

@@ -96,6 +96,8 @@ class ZRoutine:
 class ZStackBottom:
     def __init__(self):
         self.program_counter = 0  # used as a cache only
+        self.stack = []
+        self.local_vars = [0 for _ in range(15)]
 
 
 class ZStackManager: