etl::max_element has undefined behavior if the range is empty

[jonasgampigbmw]

If etl::max_element is called with an empty range, i.e. begin == end, it causes undefined behavior. According to https://en.cppreference.com/w/cpp/algorithm/max_element, it should return end instead if the range is empty.

The root cause is that begin is incremented first, which leads to begin pointing to the element after end. Then, the while-loop will execute and begin will be dereferenced.