vp9_parser: don't overwrite cached timestamps with nopts.

libavcodec/vp9_parser.c

@@ -64,7 +64,7 @@ static int parse_frame(AVCodecParserContext *ctx, const uint8_t *buf, int size)
         if (ctx->pts == AV_NOPTS_VALUE)
             ctx->pts = s->pts;
         s->pts = AV_NOPTS_VALUE;
-    } else {
+    } else if (ctx->pts != AV_NOPTS_VALUE) {
         s->pts = ctx->pts;
         ctx->pts = AV_NOPTS_VALUE;
     }