(perl #132683) don't try to convert PL_sv_placeholder into a CV

(perl #132683) don't try to convert PL_sv_placeholder into a CV

Constant folding sets PL_warnhook to PERL_WARNHOOK_FATAL, which is
&PL_sv_placeholder, an undef SV.

If warn() is called while constant folding, invoke_exception_hook()
attempts to use the value of a non-NULL PL_warnhook as a CV, which
caused an undefined value warning.

invoke_exception_hook() now treats a PL_warnhook of PERL_WARNHOOK_FATAL
the same as NULL, falling back to the normal warning handling which
throws an exception to abort constant folding.