Fix possible segfault if we try to get the main_sub from a packfile that has no constants or no main_sub
Fix possible segfault if we try to get the main_sub from a packfile that has no constants or no main_sub

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