plistlib: a <key> outside a <dict> raises IndexError instead of a proper error

Bug report

plistlib.loads on an XML plist whose <key> element appears outside a
<dict> raises an uncaught IndexError instead of a ValueError.

_PlistParser.end_key checks self.stack[-1] without first testing whether
the stack is empty:

def end_key(self):
    if self.current_key or not isinstance(self.stack[-1], dict):
        raise ValueError("unexpected key at line %d" %
                         self.parser.CurrentLineNumber)
    self.current_key = self.get_data()

When a <key> is the document's top-level element there is no enclosing
container on the stack, so self.stack[-1] raises IndexError.

Reproducer:

>>> import plistlib
>>> plistlib.loads(b"<plist><key>x</key></plist>")
Traceback (most recent call last):
  ...
IndexError: list index out of range

The sibling handler add_object (used by <integer>, <string>, etc.)
already guards the empty-stack case with elif not self.stack:, so scalar
elements at the top level are handled cleanly. A <key> outside a <dict>
should likewise raise a ValueError, consistent with the other malformed
inputs the parser rejects.

The binary plist format does not use this code path and is unaffected.

Versions

3.15 (main); the same code is present on 3.13 and 3.14.

Linked PRs

• gh-152960
• gh-152970

Beginne in plistlib bei _PlistParser.end_key und vergleiche die Stack-Behandlung mit add_object. Reproduziere den -Fall auf oberster Ebene mit plistlib.loads und überprüfe anschließend, dass eine fehlerhafte XML-Plist-Eingabe statt IndexError ValueError auslöst; die verknüpften PRs gh-152960 und gh-152970 zeigen, dass die Arbeit bereits im Gange ist.