ckDEAD: PL_curcop->cop_warnings only if PL_curcop

ckDEAD: PL_curcop->cop_warnings only if PL_curcop

RT #130621

In ckDEAD(), don't check the value of PL_curcop->cop_warnings unless
PL_curcop is non-null.

In the ticket above, the reason that PL_curcop is null is the less
than optimal way that evals free their optree: ideally the optree should
be attached to the eval CV and freed when the CV is; instead a separate
SAVEFREEOP() is done. But that fix is for another time; regardless,
ckDEAD() should have a PL_curcop != NULL guard anyway like isLEXWARN_on()
etc already do.