use version instead of some random 32bit value to switch between 16 and

32bit versions (this is more reliable, the old code failed on some files)

Originally committed as revision 10094 to svn://svn.ffmpeg.org/ffmpeg/trunk

libavcodec/4xm.c

@@ -340,7 +340,7 @@ static int decode_p_frame(FourXContext *f, uint8_t *buf, int length){
     const int stride= f->current_picture.linesize[0]>>1;
     unsigned int bitstream_size, bytestream_size, wordstream_size, extra;
 
-    if(!get32(buf-4)){
+    if(f->version){
         extra=20;
         bitstream_size= get32(buf+8);
         wordstream_size= get32(buf+12);