AAC encoder: fix possible assertion failure in PNS

Fix possible SF delta violation that would cause an
eventual assertion failure in some corner cases (esp
on very low bitrates) when marking bands for PNS due
to misuse of the sf_delta utilities

libavcodec/aaccoder.c

@@ -726,11 +726,12 @@ static void search_for_pns(AACEncContext *s, AVCodecContext *avctx, SingleChanne
                 sce->band_type[w*16+g] = NOISE_BT;
                 sce->zeroes[w*16+g] = 0;
                 prev = noise_sfi;
-            }
+            } else {
                 if (!sce->zeroes[w*16+g])
                     prev_sf = sce->sf_idx[w*16+g];
             }
         }
+    }
 }
 
 static void mark_pns(AACEncContext *s, AVCodecContext *avctx, SingleChannelElement *sce)