add -Wextra and make code warning free

[uecker]

Here is a version which compiles cleanly with -Wextra and at least some jolo tiny example also runs with -lUBsan. Not tested otherwise, so mileage may vary.

[stephanecharette]

Note that this repo has been abandoned for several years now.

The new Darknet/YOLO repo that is maintained is this one: https://github.com/hank-ai/darknet#table-of-contents

You can get more information from the Darknet/YOLO FAQ: https://www.ccoderun.ca/programming/yolo_faq/#which_repos_to_use_and_avoid

[uecker]

I know. But I have some interest in a C only version, so I share it here for people who also have a need for this.

[uecker]

Since github still seems to let me change the branch of a pull request, I close it here and reopened a new one: #8916