Merge pull request #28200 from shahkarnav115-beep:fix-avif-null-checks

imgcodecs(avif): add safety checks for AVIF decoder and encoder #28200

Add defensive checks to prevent potential null dereference in AVIF decoder and encoder paths when handling malformed input or allocation failures.

### Pull Request Readiness Checklist

See details at https://github.com/opencv/opencv/wiki/How_to_contribute#making-a-good-pull-request

- [x] I agree to contribute to the project under Apache 2 License.
- [x] To the best of my knowledge, the proposed patch is not based on a code under GPL or another license that is incompatible with OpenCV
- [x] The PR is proposed to the proper branch
- [ ] There is a reference to the original bug report and related work
- [ ] There is accuracy test, performance test and test data in opencv_extra repository, if applicable
      Patch to opencv_extra has the same branch name.
- [ ] The feature is well documented and sample code can be built with the project CMake

modules/imgcodecs/src/grfmt_avif.cpp

@@ -211,6 +211,10 @@ bool AvifDecoder::readHeader() {
     return true;
 
   decoder_ = avifDecoderCreate();
+  if (!decoder_) {
+    CV_Error(Error::StsNoMem, "Failed to create AVIF decoder");
+    return false;
+  }
   decoder_->strictFlags = AVIF_STRICT_DISABLED;
   if (!m_buf.empty()) {
     CV_Assert(m_buf.type() == CV_8UC1);
@@ -226,6 +230,11 @@ bool AvifDecoder::readHeader() {
       decoder_);
   OPENCV_AVIF_CHECK_STATUS(avifDecoderParse(decoder_), decoder_);
 
+  if (!decoder_->image) {
+    CV_Error(Error::StsParseError, "AVIF image is null after parsing");
+    return false;
+  }
+
   m_width = decoder_->image->width;
   m_height = decoder_->image->height;
   m_frame_count = decoder_->imageCount;
@@ -380,7 +389,6 @@ bool AvifEncoder::writeanimation(const Animation& animation,
                                      ? AVIF_ADD_IMAGE_FLAG_SINGLE
                                      : AVIF_ADD_IMAGE_FLAG_NONE;
   std::vector<AvifImageUniquePtr> images;
-  std::vector<cv::Mat> imgs_scaled;
   for (const cv::Mat &img : animation.frames) {
     CV_CheckType(
         img.type(),
@@ -388,11 +396,17 @@ bool AvifEncoder::writeanimation(const Animation& animation,
             ((bit_depth == 10 || bit_depth == 12) && img.depth() == CV_16U),
         "AVIF only supports bit depth of 8 with CV_8U input or "
         "bit depth of 10 or 12 with CV_16U input");
+
     CV_Check(img.channels(),
              img.channels() == 1 || img.channels() == 3 || img.channels() == 4,
              "AVIF only supports 1, 3, 4 channels");
 
-    images.emplace_back(ConvertToAvif(img, do_lossless, bit_depth, m_metadata));
+    auto avifImg = ConvertToAvif(img, do_lossless, bit_depth, m_metadata);
+    if (!avifImg) {
+        CV_Error(Error::StsError, "Failed to convert Mat to AVIF image");
+        return false;
+    }
+    images.emplace_back(std::move(avifImg));
   }
 
   for (size_t i = 0; i < images.size(); i++)