C: Do not store invalid GvCV, only cv or &sv_list entries

Fixes #313, where Errno::ECHILD stored lexwarn1 as GvCV, which
subsequently could not be globally destructed and SEGV with -O3
t/issue240.t: not repro anymore with HARNESS_ACTIVE either
t/issue282.t: unTODO on linux (unsafe use-after-free not detected there)