aacpsy: calc_reduction_3gpp() handle active_lines = 0

Signed-off-by: Michael Niedermayer <michaelni@gmx.at>

libavcodec/aacpsy.c

@@ -526,6 +526,9 @@ static float calc_reduction_3gpp(float a, float desired_pe, float pe,
 {
     float thr_avg, reduction;
 
+    if(active_lines == 0.0)
+        return 0;
+
     thr_avg   = exp2f((a - pe) / (4.0f * active_lines));
     reduction = exp2f((a - desired_pe) / (4.0f * active_lines)) - thr_avg;