bundlerepo: avoid exception in __del__ when the bundle doesn't exist
$ hg -R bundle://foo.hg
abort: No such file or directory: foo.hg
Exception exceptions.AttributeError: "'bundlerepository' object has no attribute 'bundlefile'" in <bound method bundlerepository.__del__ of <mercurial.bundlerepo.bundlerepository object at 0xa7ab9fac>> ignored
abort: Failed to parse .../t/.hg/hgrc
File contains no section headers.
file: .../t/.hg/hgrc, line: 1
'invalid\n'