Handle object 0 (nothing) gracefully in object parser

Per the Z-machine spec, object 0 means "nothing" and operations on it
should be safe no-ops: get_child/get_sibling/get_parent return 0,
test_attr returns false, set/clear_attr are no-ops, etc. Previously
these threw ZObjectIllegalObjectNumber, crashing on games like Curses
that pass object 0 to get_child during room transitions.

src/mudlib/zmachine/zobjectparser.py

@@ -163,6 +163,8 @@ class ZObjectParser:
     def get_attribute(self, objectnum, attrnum):
         """Return value (0 or 1) of attribute number ATTRNUM of object
         number OBJECTNUM."""
+        if objectnum == 0:
+            return 0
 
         object_addr = self._get_object_addr(objectnum)
 
@@ -183,6 +185,8 @@ class ZObjectParser:
 
     def set_attribute(self, objectnum, attrnum):
         """Set attribute number ATTRNUM of object number OBJECTNUM to 1."""
+        if objectnum == 0:
+            return
 
         object_addr = self._get_object_addr(objectnum)
 
@@ -206,6 +210,8 @@ class ZObjectParser:
 
     def clear_attribute(self, objectnum, attrnum):
         """Clear attribute number ATTRNUM of object number OBJECTNUM to 0."""
+        if objectnum == 0:
+            return
 
         object_addr = self._get_object_addr(objectnum)
 
@@ -247,18 +253,24 @@ class ZObjectParser:
 
     def get_parent(self, objectnum):
         """Return object number of parent of object number OBJECTNUM."""
+        if objectnum == 0:
+            return 0
 
         [parent, sibling, child] = self._get_parent_sibling_child(objectnum)
         return parent
 
     def get_child(self, objectnum):
         """Return object number of child of object number OBJECTNUM."""
+        if objectnum == 0:
+            return 0
 
         [parent, sibling, child] = self._get_parent_sibling_child(objectnum)
         return child
 
     def get_sibling(self, objectnum):
         """Return object number of sibling of object number OBJECTNUM."""
+        if objectnum == 0:
+            return 0
 
         [parent, sibling, child] = self._get_parent_sibling_child(objectnum)
         return sibling
@@ -298,6 +310,8 @@ class ZObjectParser:
 
     def remove_object(self, objectnum):
         """Detach object OBJECTNUM from its parent (unlink from sibling chain)."""
+        if objectnum == 0:
+            return
 
         parent = self.get_parent(objectnum)
         if parent == 0:
@@ -335,6 +349,8 @@ class ZObjectParser:
         Per the Z-spec: if new_child already has a parent, it is first
         removed from that parent's child list, then made the first child
         of parent_object."""
+        if parent_object == 0 or new_child == 0:
+            return
 
         # Remove from old parent first (spec says "first removed")
         self.remove_object(new_child)
@@ -347,6 +363,8 @@ class ZObjectParser:
 
     def get_shortname(self, objectnum):
         """Return 'short name' of object number OBJECTNUM as ascii string."""
+        if objectnum == 0:
+            return ""
 
         addr = self._get_proptable_addr(objectnum)
         return self._stringfactory.get(addr + 1)
@@ -354,6 +372,8 @@ class ZObjectParser:
     def get_prop(self, objectnum, propnum):
         """Return either a byte or word value of property PROPNUM of
         object OBJECTNUM."""
+        if objectnum == 0:
+            return 0
         (addr, size) = self.get_prop_addr_len(objectnum, propnum)
         if size == 1:
             return self._memory[addr]
@@ -467,6 +487,8 @@ class ZObjectParser:
     def get_property_data_address(self, objectnum, propnum):
         """Return the address of property PROPNUM's data bytes for object
         OBJECTNUM. Return 0 if the object doesn't have that property."""
+        if objectnum == 0:
+            return 0
 
         try:
             addr, size = self.get_prop_addr_len(objectnum, propnum)
@@ -484,6 +506,8 @@ class ZObjectParser:
         """If PROPNUM is 0, return the first property number of object OBJECTNUM.
         Otherwise, return the property number after PROPNUM in the property list.
         Return 0 if there are no more properties."""
+        if objectnum == 0:
+            return 0
 
         if propnum == 0:
             # Return first property number