aacdec_usac: skip coeff decoding if the number to be decoded is 0

Yet another thing not mentioned in the spec.

libavcodec/aac/aacdec_usac.c

@@ -572,9 +572,15 @@ static int decode_spectrum_and_dequant_ac(AACDecContext *s, float coef[1024],
     int gb_count;
     GetBitContext gb2;
 
-    ff_aac_ac_init(&ac, gb);
     c = ff_aac_ac_map_process(state, reset, N);
 
+    if (!len) {
+        ff_aac_ac_finish(state, 0, N);
+        return 0;
+    }
+
+    ff_aac_ac_init(&ac, gb);
+
     /* Backup reader for rolling back by 14 bits at the end */
     gb2 = *gb;
     gb_count = get_bits_count(&gb2);