nuv: check RTjpeg header for validity

CC: libav-stable@libav.org
(cherry picked from commit 859a579e9b)

Signed-off-by: Anton Khirnov <anton@khirnov.net>

libavcodec/rtjpeg.h

@@ -25,6 +25,9 @@
 #include <stdint.h>
 #include "dsputil.h"
 
+#define RTJPEG_FILE_VERSION 0
+#define RTJPEG_HEADER_SIZE 12
+
 typedef struct {
     int w, h;
     DSPContext *dsp;