fix: translate RecursionError to StackError in fallback Unpacker.skip()

skip() was the only unpack entry point in fallback.py that let a
RecursionError escape on deeply nested input; unpack() and __next__()
both re-raise it as StackError, and the C extension raises StackError
on this path too. Callers guarding against adversarial nesting with
except StackError/ValueError were unprotected when skipping.

Wrap the body the same way unpack() does, leaving _consume() outside
the try so the post-failure buffer state matches.

Author: hdimer

msgpack/fallback.py

@@ -582,7 +582,10 @@ def __next__(self):
     next = __next__
 
     def skip(self):
-        self._unpack(EX_SKIP)
+        try:
+            self._unpack(EX_SKIP)
+        except RecursionError:
+            raise StackError
         self._consume()
 
     def unpack(self):

test/test_except.py

@@ -97,6 +97,11 @@ def test_invalidvalue():
     with raises(StackError):
         unpackb(b"\x91" * 3000)  # nested fixarray(len=1)
 
+    with raises(StackError):
+        unpacker = Unpacker()
+        unpacker.feed(b"\x91" * 3000)
+        unpacker.skip()
+
 
 def test_no_memory_leak_on_nested_invalid_tag() -> None:
     """Regression test: unpacking nested arrays containing an invalid tag must not leak objects."""