doc/examples/decode_video: Fix format string vulnerability

Fixes: CID1404843

Signed-off-by: Michael Niedermayer <michael@niedermayer.cc>
(cherry picked from commit 23edd41a0d)
Signed-off-by: Michael Niedermayer <michael@niedermayer.cc>

doc/examples/decode_video.c

@@ -64,7 +64,7 @@ static int decode_write_frame(const char *outfilename, AVCodecContext *avctx,
         fflush(stdout);
 
         /* the picture is allocated by the decoder, no need to free it */
-        snprintf(buf, sizeof(buf), outfilename, *frame_count);
+        snprintf(buf, sizeof(buf), "%s-%d", outfilename, *frame_count);
         pgm_save(frame->data[0], frame->linesize[0],
                  frame->width, frame->height, buf);
         (*frame_count)++;