libavcodec/util: Fix timebase overflow check

It could accidentally divide by zero if num was zero.
Signed-off-by: Derek Buitenhuis <derek.buitenhuis@gmail.com>

libavcodec/utils.c

@@ -1475,7 +1475,7 @@ FF_ENABLE_DEPRECATION_WARNINGS
         if (!avctx->rc_initial_buffer_occupancy)
             avctx->rc_initial_buffer_occupancy = avctx->rc_buffer_size * 3 / 4;
 
-        if (avctx->ticks_per_frame &&
+        if (avctx->ticks_per_frame && avctx->time_base.num &&
             avctx->ticks_per_frame > INT_MAX / avctx->time_base.num) {
             av_log(avctx, AV_LOG_ERROR,
                    "ticks_per_frame %d too large for the timebase %d/%d.",