Make av_set_pts_info keep previous time base if new one is invalid.

Fixes issue 2475.
Signed-off-by: Ronald S. Bultje <rsbultje@gmail.com>
(cherry picked from commit b3190529)

libavformat/avformat.h

@@ -1267,7 +1267,8 @@ AVChapter *ff_new_chapter(AVFormatContext *s, int id, AVRational time_base,
                           int64_t start, int64_t end, const char *title);
 
 /**
- * Set the pts for a given stream.
+ * Set the pts for a given stream. If the new values would be invalid
+ * (<= 0), it leaves the AVStream unchanged.
  *
  * @param s stream
  * @param pts_wrap_bits number of bits effectively used by the pts

libavformat/utils.c

@@ -3762,16 +3762,19 @@ int ff_hex_to_data(uint8_t *data, const char *p)
 void av_set_pts_info(AVStream *s, int pts_wrap_bits,
                      unsigned int pts_num, unsigned int pts_den)
 {
-    s->pts_wrap_bits = pts_wrap_bits;
-
-    if(av_reduce(&s->time_base.num, &s->time_base.den, pts_num, pts_den, INT_MAX)){
-        if(s->time_base.num != pts_num)
-            av_log(NULL, AV_LOG_DEBUG, "st:%d removing common factor %d from timebase\n", s->index, pts_num/s->time_base.num);
+    AVRational new_tb;
+    if(av_reduce(&new_tb.num, &new_tb.den, pts_num, pts_den, INT_MAX)){
+        if(new_tb.num != pts_num)
+            av_log(NULL, AV_LOG_DEBUG, "st:%d removing common factor %d from timebase\n", s->index, pts_num/new_tb.num);
     }else
         av_log(NULL, AV_LOG_WARNING, "st:%d has too large timebase, reducing\n", s->index);
 
-    if(!s->time_base.num || !s->time_base.den)
-        s->time_base.num= s->time_base.den= 0;
+    if(new_tb.num <= 0 || new_tb.den <= 0) {
+        av_log(NULL, AV_LOG_ERROR, "Ignoring attempt to set invalid timebase for st:%d\n", s->index);
+        return;
+    }
+    s->time_base = new_tb;
+    s->pts_wrap_bits = pts_wrap_bits;
 }
 
 int ff_url_join(char *str, int size, const char *proto,