add file existence check in yolo_console_dll.cpp

[RRRChangeche]

I'm trying to use C++ API to integrate darknet with my C++ project on windows. I followed the tutorial on this topic in README.md.

During the test toward my input image files (with #define OPENCV), I found that whenever I gave the wrong file path, the console crashed because the given file path did not exist. To be more convenient for input image files testing, I implemented the file existence check section in yolo_console_dll.cpp.

[cenit]

the fix is not portable on windows, and so not acceptable. I am sorry. You could try to rework it with the filesystem library from c++17, maybe