Fixed LGTM warnings (Tencent#3891)

src/stb_image.h
src/layer/detectionoutput.cpp
tools/quantize/imreadwrite.cpp

Author: proydakov

src/layer/detectionoutput.cpp

@@ -170,7 +170,7 @@ int DetectionOutput::forward(const std::vector<Mat>& bottom_blobs, std::vector<M
     for (int i = 0; i < num_prior; i++)
     {
         // if score of background class is larger than confidence threshold
-        float score = mxnet_ssd_style ? confidence[i] : confidence[i * num_class_copy];
+        float score = mxnet_ssd_style ? confidence[i] : confidence[static_cast<size_t>(i) * static_cast<size_t>(num_class_copy)];
         if (score >= (1.0 - confidence_threshold))
         {
             continue;

src/stb_image.h

@@ -1756,7 +1756,8 @@ static stbi__uint16 *stbi__convert_format16(stbi__uint16 *data, int img_n, int r
    if (req_comp == img_n) return data;
    STBI_ASSERT(req_comp >= 1 && req_comp <= 4);
 
-   good = (stbi__uint16 *) stbi__malloc(req_comp * x * y * 2);
+   const size_t alloc_size = static_cast<size_t>(req_comp) * static_cast<size_t>(x) * static_cast<size_t>(y) * 2;
+   good = (stbi__uint16 *) stbi__malloc(alloc_size);
    if (good == NULL) {
       STBI_FREE(data);
       return (stbi__uint16 *) stbi__errpuc("outofmem", "Out of memory");

tools/quantize/imreadwrite.cpp

@@ -86,7 +86,7 @@ Mat imread(const std::string& path, int flags)
         return Mat();
     }
 
-    memcpy(img.data, pixeldata, w * h * c);
+    memcpy(img.data, pixeldata, static_cast<size_t>(w) * static_cast<size_t>(h) * static_cast<size_t>(c));
 
     stbi_image_free(pixeldata);