lavfi/atempo: Avoid false triggering an assertion failure

Steps to reproduce:
1. revert 4240e5b0
2. ./ffmpeg -f lavfi -i sine=d=1 -af aselect=e=0,atempo=0.5 -y atempo.wav
Signed-off-by: Marton Balint <cus@passwd.hu>

libavfilter/af_atempo.c

@@ -914,6 +914,11 @@ static int yae_flush(ATempoContext *atempo,
 
     atempo->state = YAE_FLUSH_OUTPUT;
 
+    if (!atempo->nfrag) {
+        // there is nothing to flush:
+        return 0;
+    }
+
     if (atempo->position[0] == frag->position[0] + frag->nsamples &&
         atempo->position[1] == frag->position[1] + frag->nsamples) {
         // the current fragment is already flushed: