mess_sv(): access only if PL_curcop is non-null

mess_sv(): access only if PL_curcop is non-null

RT #130621

In Perl_mess_sv(), don't try to add an "at foo line NN" to the error
message if PL_curcop is 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,
mess_sv() should have a PL_curcop != NULL guard anyway.