Fix crash

I'm not enough of an expert on the gzip format to understand why this caused a crash, this code seems to have fixed it. I followed http://commandlinefanatic.com/cgi-bin/showarticle.cgi?article=art001

        // This is a back-pointer to a position in the stream
        // Interpret the length here as specified in 3.2.5
        if ( node->code < 265 )
        {
          length = node->code - 254;
        }
        else
        {
          if ( node->code < 285 )
          {
            extra_bits = read_bits_inv( stream, ( node->code - 261 ) / 4 );
            length = extra_bits + extra_length_addend[ node->code - 265 ];
          }
          else
          {
            length = 258;
          }
        }