lavc/hevc: check framerate num/den to be strictly positive

Rather than just != 0. These values are read as uint32 and can become
negative when cast to int.

libavcodec/hevc/hevcdec.c

@@ -392,7 +392,7 @@ static void export_stream_params(HEVCContext *s, const HEVCSPS *sps)
         den = sps->vui.vui_time_scale;
     }
 
-    if (num != 0 && den != 0)
+    if (num > 0 && den > 0)
         av_reduce(&avctx->framerate.den, &avctx->framerate.num,
                   num, den, 1 << 30);
 }

libavcodec/hevc/parser.c

@@ -101,7 +101,7 @@ static int hevc_parse_slice_header(AVCodecParserContext *s, H2645NAL *nal,
         den = sps->vui.vui_time_scale;
     }
 
-    if (num != 0 && den != 0)
+    if (num > 0 && den > 0)
         av_reduce(&avctx->framerate.den, &avctx->framerate.num,
                   num, den, 1 << 30);