trace: protect from more null ptr traces
trace: protect from more null ptr traces

i.e. fixes ./parrot_old -t t/pmc/resizablefloatarray.t

tkob/parrot,tkob/parrot,youprofit/parrot,tkob/parrot,tkob/parrot,tkob/parrot,parrot/parrot,youprofit/parrot,youprofit/parrot,parrot/parrot,youprofit/parrot,youprofit/parrot,youprofit/parrot,tkob/parrot,tkob/parrot,tkob/parrot,parrot/parrot,youprofit/parrot,parrot/parrot,parrot/parrot,youprofit/parrot