Correct pulse amplitude application - a negative or 0 coefficient implies the

pulse is subtracted, else it is added. Also avoid a divide by 0.

Based on a patch by Alex Converse (alex converse gmail com)
Fixes part of issue632

Originally committed as revision 15294 to svn://svn.ffmpeg.org/ffmpeg/trunk

libavcodec/aac.c

@@ -753,7 +753,9 @@ static int decode_spectrum_and_dequant(AACContext * ac, float coef[1024], GetBit
     if (pulse_present) {
         for(i = 0; i < pulse->num_pulse; i++){
             float co  = coef_base[ pulse->pos[i] ];
-            float ico = co / sqrtf(sqrtf(fabsf(co))) + pulse->amp[i];
+            float ico = -pulse->amp[i];
+            if (co)
+                ico = co / sqrtf(sqrtf(fabsf(co))) + (co > 0 ? -ico : ico);
             coef_base[ pulse->pos[i] ] = cbrtf(fabsf(ico)) * ico;
         }
     }