Added check for integer overflow in get_num_images (#1397)

Added check for integer overflow in get_num_images  (#1397)

As discussed in pull request 1396, added a check for integer overflow.
Change list:
Defined num_images as unsigned int
Moved the if statement to check for an empty directory to the beginning of the read directory section
Added a check to see if num images would roll back to zero when incrementing.