Fix a GC-related bug in Parrot_str_clone
Fix a GC-related bug in Parrot_str_clone

Using STRUCT_COPY and changing the flags afterwards can create an invalid
string header. If Parrot_gc_allocate_string_storage then triggers a GC run,
bad things can happen like external strings being compacted.

git-svn-id: 6e74a02f85675cec270f5d931b0f6998666294a3@48808 d31e2699-5ff4-0310-a27c-f18f2fbe73fe

gitster/parrot,FROGGS/parrot,FROGGS/parrot,gitster/parrot,fernandobrito/parrot,tewk/parrot-select,fernandobrito/parrot,gitster/parrot,tkob/parrot,tkob/parrot,parrot/parrot,tewk/parrot-select,fernandobrito/parrot,FROGGS/parrot,youprofit/parrot,tewk/parrot-select,fernandobrito/parrot,gagern/parrot,gitster/parrot,FROGGS/parrot,tkob/parrot,gagern/parrot,gagern/parrot,tkob/parrot,youprofit/parrot,gitster/parrot,parrot/parrot,gagern/parrot,youprofit/parrot,tkob/parrot,youprofit/parrot,gagern/parrot,gagern/parrot,fernandobrito/parrot,tewk/parrot-select,parrot/parrot,tkob/parrot,parrot/parrot,parrot/parrot,tkob/parrot,FROGGS/parrot,youprofit/parrot,fernandobrito/parrot,youprofit/parrot,FROGGS/parrot,gitster/parrot,gitster/parrot,fernandobrito/parrot,tewk/parrot-select,FROGGS/parrot,tkob/parrot,youprofit/parrot,tewk/parrot-select,FROGGS/parrot,tewk/parrot-select,youprofit/parrot,gagern/parrot