doc/examples/decode_video: Fix format string vulnerability

Fixes: CID1404843
Signed-off-by: Michael Niedermayer <michael@niedermayer.cc>

doc/examples/decode_video.c

@@ -74,7 +74,7 @@ static void decode(AVCodecContext *dec_ctx, AVFrame *frame, AVPacket *pkt,
 
         /* the picture is allocated by the decoder. no need to
            free it */
-        snprintf(buf, sizeof(buf), filename, dec_ctx->frame_number);
+        snprintf(buf, sizeof(buf), "%s-%d", filename, dec_ctx->frame_number);
         pgm_save(frame->data[0], frame->linesize[0],
                  frame->width, frame->height, buf);
     }