mtree: return early from walk() if the path does not exist

Author: arichardson

pycheribuild/mtree.py

@@ -216,6 +216,8 @@ def _walk(self, top, prefix) -> "Iterator[tuple[MtreePath, list[str], list[str]]
             yield from v._walk(MtreePath(), prefix)
 
     def walk(self, top) -> "Iterator[tuple[MtreePath, list[str], list[str]]]":
+        if top not in self.children:
+            return iter([])
         return self._walk(top, MtreePath())