fix mp3in segfault when skip=0
fix mp3in segfault when skip=0

ketchupok/csound,csound/csound,ketchupok/csound,ketchupok/csound,csound/csound,ketchupok/csound,csound/csound,ketchupok/csound,csound/csound,ketchupok/csound,csound/csound,csound/csound,ketchupok/csound,csound/csound,csound/csound,ketchupok/csound,ketchupok/csound,csound/csound,csound/csound,ketchupok/csound