avcodec/utils: fix time_base after the framerate addition

time_base != 1/framerate for example for interlaced video
Signed-off-by: Michael Niedermayer <michaelni@gmx.at>

libavcodec/utils.c

@@ -1677,7 +1677,7 @@ int attribute_align_arg avcodec_open2(AVCodecContext *avctx, const AVCodec *code
 
 #if FF_API_AVCTX_TIMEBASE
         if (avctx->framerate.num > 0 && avctx->framerate.den > 0)
-            avctx->time_base = av_inv_q(avctx->framerate);
+            avctx->time_base = av_inv_q(av_mul_q(avctx->framerate, (AVRational){avctx->ticks_per_frame, 1}));
 #endif
     }
 end:
@@ -2391,7 +2391,7 @@ fail:
 
 #if FF_API_AVCTX_TIMEBASE
     if (avctx->framerate.num > 0 && avctx->framerate.den > 0)
-        avctx->time_base = av_inv_q(avctx->framerate);
+        avctx->time_base = av_inv_q(av_mul_q(avctx->framerate, (AVRational){avctx->ticks_per_frame, 1}));
 #endif
 
     return ret;