add input paramter checking that verifies the existance of the input files to stero_match example

Hi,
I'm just learning the git process. I added small improvement to one of the example and want to submit a pull request. Hopefully, this is useful and gets accepted.

Thanks!
I believe it's better to check `img1.empty()` and `img2.empty()` (see lines below), because I can pass `file1.txt` and `file2.txt` as command line parameters, but the images will be empty