Undefined lex sub used as inherited method crashes

Thanks to misuse of CvGV.

$ perl5.19.5 -XMfeature=:all -e 'my sub foo {} undef &foo; *UNIVERSAL::bar = \&foo; main->bar()'
Segmentation fault: 11

When deciding under which name to autoload the sub, we can’t assume
CvGV(that_sub) is a GV, as it may be null.