aacps: avoid division by zero in stereo_processing

This fixes a SIGFPE crash in the aac_fixed decoder.
Signed-off-by: Andreas Cadhalpun <Andreas.Cadhalpun@googlemail.com>
Reviewed-by: Rostislav Pehlivanov <atomnuker@gmail.com>

libavcodec/aacps.c

@@ -940,7 +940,7 @@ static void stereo_processing(PSContext *ps, INTFLOAT (*l)[32][2], INTFLOAT (*r)
             LOCAL_ALIGNED_16(INTFLOAT, h_step, [2], [4]);
             int start = ps->border_position[e];
             int stop  = ps->border_position[e+1];
-            INTFLOAT width = Q30(1.f) / (stop - start);
+            INTFLOAT width = Q30(1.f) / ((stop - start) ? (stop - start) : 1);
 #if USE_FIXED
             width <<= 1;
 #endif