Avoid reading out of bounds due to negative aaIccIndexMapped

Fixes: 3452/clusterfuzz-testcase-4898065225875456

Found-by: continuous fuzzing process https://github.com/google/oss-fuzz/tree/master/projects/ffmpeg

Author: mstorsjo

libSBRdec/src/psdec.cpp

@@ -944,7 +944,7 @@ void initSlotBasedRotation( HANDLE_PS_DEC h_ps_d, /*!< pointer to the module sta
 
   FIXP_SGL  invL;
   FIXP_DBL  ScaleL, ScaleR;
-  FIXP_DBL  Alpha, Beta;
+  FIXP_DBL  Alpha, Beta, AlphasValue;
   FIXP_DBL  h11r, h12r, h21r, h22r;
 
   const FIXP_DBL  *PScaleFactors;
@@ -1015,8 +1015,11 @@ void initSlotBasedRotation( HANDLE_PS_DEC h_ps_d, /*!< pointer to the module sta
     ScaleR = PScaleFactors[noIidSteps + h_ps_d->specificTo.mpeg.coef.aaIidIndexMapped[env][bin]];
     ScaleL = PScaleFactors[noIidSteps - h_ps_d->specificTo.mpeg.coef.aaIidIndexMapped[env][bin]];
 
-    Beta   = fMult (fMult( Alphas[h_ps_d->specificTo.mpeg.coef.aaIccIndexMapped[env][bin]], ( ScaleR - ScaleL )), FIXP_SQRT05);
-    Alpha  = Alphas[h_ps_d->specificTo.mpeg.coef.aaIccIndexMapped[env][bin]]>>1;
+    AlphasValue = 0;
+    if (h_ps_d->specificTo.mpeg.coef.aaIccIndexMapped[env][bin] >= 0)
+      AlphasValue = Alphas[h_ps_d->specificTo.mpeg.coef.aaIccIndexMapped[env][bin]];
+    Beta   = fMult (fMult( AlphasValue, ( ScaleR - ScaleL )), FIXP_SQRT05);
+    Alpha  = AlphasValue>>1;
 
     /* Alpha and Beta are now both scaled by 2 shifts right */