add test/data for video support : Fix test issue

include/exiv2/riffvideo.hpp

@@ -53,8 +53,8 @@ class EXIV2API RiffVideo : public Image {
 
  protected:
   class HeaderReader {
-    std::string id_;
-    uint64_t size_;
+    std::string id_ = "";
+    uint64_t size_ = 0;
 
    public:
     explicit HeaderReader(BasicIo::UniquePtr& io);

src/helper_functions.cpp

@@ -64,7 +64,7 @@ std::string readStringWcharTag(BasicIo::UniquePtr& io, size_t length) {
 std::string readStringTag(BasicIo::UniquePtr& io, size_t length) {
   Internal::enforce(length <= io->size() - io->tell(), Exiv2::ErrorCode::kerCorruptedMetadata);
   DataBuf FieldBuf = io->read(length);
-  return Exiv2::toString(FieldBuf.data());
+  return Exiv2::toString(FieldBuf.data()).substr(0, length);
 }
 
 }  // namespace Exiv2