guided filter: properly validate radius parameter before calling boxblur

rtengine/boxblur.h

@@ -35,8 +35,6 @@ namespace rtengine
 template<class T, class A> void boxblur (T** src, A** dst, int radx, int rady, int W, int H)
 {
     //box blur image; box range = (radx,rady)
-    radx = min(radx, W-1);
-    rady = min(rady, H-1);
 
     AlignedBuffer<float>* buffer = new AlignedBuffer<float> (W * H);
     float* temp = buffer->data;

rtengine/guidedfilter.cc

@@ -110,6 +110,7 @@ void guidedFilter(const array2D<float> &guide, const array2D<float> &src, array2
     const auto f_mean =
         [](array2D<float> &d, array2D<float> &s, int rad) -> void
         {
+            rad = min(rad, s.width() / 2, s.height() / 2);
             boxblur<float, float>(s, d, rad, rad, s.width(), s.height());
         };