PC modification in gc_event_hook_body was careless.  There are (so
to say) abnormal iseqs stored in the cfp.  We have to check sanity
before we touch the PC.

This has not been fixed because there was no way to (ab)use the
setup from pure-Ruby.  However by using our official C APIs it is
possible to touch such frame(s), resulting in SEGV.

Fixes [Bug #14834].