Fix two zmachine bugs found during code audit

quetzal.py: typo in _parse_umem() — cmem.dynamic_start should be
cmem._dynamic_start. Would crash on uncompressed memory restore.

zcpu.py: op_erase_window had if/if/else instead of if/elif/else.
When window_number was -1, the second if fell through to else,
calling erase_window(-1) after the correct reset path.

src/mudlib/zmachine/quetzal.py

@@ -172,7 +172,7 @@ class QuetzalParser:
         self._seen_mem_or_stks = True
 
         cmem = self._zmachine._mem
-        dynamic_len = (cmem._dynamic_end - cmem.dynamic_start) + 1
+        dynamic_len = (cmem._dynamic_end - cmem._dynamic_start) + 1
         log(f"  Dynamic memory length is {dynamic_len}")
         self._last_loaded_metadata["dynamic memory length"] = dynamic_len
 

src/mudlib/zmachine/zcpu.py

@@ -804,7 +804,7 @@ class ZCpu:
         if window_number == -1:
             self.op_split_window(0)
             self._ui.screen.erase_window(zscreen.WINDOW_LOWER)
-        if window_number == -2:
+        elif window_number == -2:
             self._ui.screen.erase_window(zscreen.WINDOW_LOWER)
             self._ui.screen.erase_window(zscreen.WINDOW_UPPER)
         else: