[core] Fix `--compat-opt allow-unsafe-ext` (#10336)

Fixes bug in 5ce582448e

Authored by: bashonly, rdamas

Co-authored-by: Robert Damas <robert.damas@byom.de>

yt_dlp/__init__.py

@@ -599,7 +599,7 @@ def validate_options(opts):
         warnings.append(
             'Using allow-unsafe-ext opens you up to potential attacks. '
             'Use with great care!')
-        _UnsafeExtensionError.sanitize_extension = lambda x: x
+        _UnsafeExtensionError.sanitize_extension = lambda x, prepend=False: x
 
     return warnings, deprecation_warnings