ffprobe: report unavailable SAR correctly in stream info

av_guess_sample_aspect_ratio() will return undefined or missing
value as {0,1}. This fixes show_stream() to check numerator to
display 'N/A' when appropriate. show_frame() does this already
correctly.
Signed-off-by: Timo Teräs <timo.teras@iki.fi>
(cherry picked from commit c663dce0)

fftools/ffprobe.c

@@ -2521,7 +2521,7 @@ static int show_stream(WriterContext *w, AVFormatContext *fmt_ctx, int stream_id
 #endif
         print_int("has_b_frames", par->video_delay);
         sar = av_guess_sample_aspect_ratio(fmt_ctx, stream, NULL);
-        if (sar.den) {
+        if (sar.num) {
             print_q("sample_aspect_ratio", sar, ':');
             av_reduce(&dar.num, &dar.den,
                       par->width  * sar.num,