examples/demuxing_decoding: use properties from frame instead of video_dec_ctx

This is more robust.

And only check if there is actually a frame returned.
Reviewed-by: Michael Niedermayer <michaelni@gmx.at>
Signed-off-by: Andreas Cadhalpun <Andreas.Cadhalpun@googlemail.com>

doc/examples/demuxing_decoding.c

@@ -81,8 +81,11 @@ static int decode_packet(int *got_frame, int cached)
             fprintf(stderr, "Error decoding video frame (%s)\n", av_err2str(ret));
             return ret;
         }
-        if (video_dec_ctx->width != width || video_dec_ctx->height != height ||
-            video_dec_ctx->pix_fmt != pix_fmt) {
+
+        if (*got_frame) {
+
+            if (frame->width != width || frame->height != height ||
+                frame->format != pix_fmt) {
                 /* To handle this change, one could call av_image_alloc again and
                  * decode the following frames into another rawvideo file. */
                 fprintf(stderr, "Error: Width, height and pixel format have to be "
@@ -91,12 +94,11 @@ static int decode_packet(int *got_frame, int cached)
                         "old: width = %d, height = %d, format = %s\n"
                         "new: width = %d, height = %d, format = %s\n",
                         width, height, av_get_pix_fmt_name(pix_fmt),
-                    video_dec_ctx->width, video_dec_ctx->height,
-                    av_get_pix_fmt_name(video_dec_ctx->pix_fmt));
+                        frame->width, frame->height,
+                        av_get_pix_fmt_name(frame->format));
                 return -1;
             }
 
-        if (*got_frame) {
             printf("video_frame%s n:%d coded_n:%d pts:%s\n",
                    cached ? "(cached)" : "",
                    video_frame_count++, frame->coded_picture_number,