Fix import crash

I was trying to import a solid colored metal/roughness texture that
worked in Godot 4.1.1 but failed to load on master. I eventually tracked
it down to this: to_x and to_y can be 0, which then leads them to be -1
later, which then causes ofs to overflow.

Some of these cases were hidden because of the use of Vector so we were
just happily scribbling in ram elsewhere.

Switched to LocalVector as it made debugging a bit easier.

core/io/image.cpp

@@ -1930,8 +1930,7 @@ Error Image::generate_mipmaps(bool p_renormalize) {
 }
 
 Error Image::generate_mipmap_roughness(RoughnessChannel p_roughness_channel, const Ref<Image> &p_normal_map) {
-	Vector<double> normal_sat_vec; //summed area table
-	double *normal_sat = nullptr; //summed area table for normal map
+	LocalVector<double> normal_sat_vec; //summed area table
 	int normal_w = 0, normal_h = 0;
 
 	ERR_FAIL_COND_V_MSG(p_normal_map.is_null() || p_normal_map->is_empty(), ERR_INVALID_PARAMETER, "Must provide a valid normal map for roughness mipmaps");
@@ -1945,8 +1944,7 @@ Error Image::generate_mipmap_roughness(RoughnessChannel p_roughness_channel, con
 	normal_h = nm->get_height();
 
 	normal_sat_vec.resize(normal_w * normal_h * 3);
-
-	normal_sat = normal_sat_vec.ptrw();
+	double *normal_sat = normal_sat_vec.ptr();
 
 	//create summed area table
 
@@ -2021,24 +2019,26 @@ Error Image::generate_mipmap_roughness(RoughnessChannel p_roughness_channel, con
 					avg[2] += normal_sat[tofs + 2];
 				}
 
-				if (from_y > 0) {
+				if (from_y > 0 && to_x > 0) {
 					uint32_t tofs = ((from_y - 1) * normal_w + to_x) * 3;
 					avg[0] -= normal_sat[tofs + 0];
 					avg[1] -= normal_sat[tofs + 1];
 					avg[2] -= normal_sat[tofs + 2];
 				}
 
-				if (from_x > 0) {
+				if (from_x > 0 && to_y > 0) {
 					uint32_t tofs = (to_y * normal_w + (from_x - 1)) * 3;
 					avg[0] -= normal_sat[tofs + 0];
 					avg[1] -= normal_sat[tofs + 1];
 					avg[2] -= normal_sat[tofs + 2];
 				}
 
-				uint32_t tofs = (to_y * normal_w + to_x) * 3;
-				avg[0] += normal_sat[tofs + 0];
-				avg[1] += normal_sat[tofs + 1];
-				avg[2] += normal_sat[tofs + 2];
+				if (to_y > 0 && to_x > 0) {
+					uint32_t tofs = (to_y * normal_w + to_x) * 3;
+					avg[0] += normal_sat[tofs + 0];
+					avg[1] += normal_sat[tofs + 1];
+					avg[2] += normal_sat[tofs + 2];
+				}
 
 				double div = double(size_x * size_y);
 				Vector3 vec(avg[0] / div, avg[1] / div, avg[2] / div);