lavf/img2dec: Reduce the probe score for incomplete jpgs.

Ensures that probing doesn't finish prematurely for small files.

libavformat/img2dec.c

@@ -756,7 +756,7 @@ static int jpeg_probe(AVProbeData *p)
     if (state == EOI)
         return AVPROBE_SCORE_EXTENSION + 1;
     if (state == SOS)
-        return AVPROBE_SCORE_EXTENSION / 2 + 1;
+        return AVPROBE_SCORE_EXTENSION / 2;
     return AVPROBE_SCORE_EXTENSION / 8;
 }