Fixes issue with one/two images failing

If only one image was present, it was failing on the second loop.
With two images, it showed the first image only on the first pass
and the second image only after that. Resolves this issue.

Author: kattni

adafruit_slideshow.py

@@ -283,15 +283,15 @@ def advance(self):
             elif not self.loop:
                 return False
             else:
-                image_count = len(self._file_list) - 1
+                image_count = len(self._file_list)
                 if self._current_image < 0:
                     self._current_image += image_count
-                elif self._current_image > image_count:
+                elif self._current_image >= image_count:
                     self._current_image -= image_count
                 self._reorder_images()
 
-            imagename = self._file_list[self._current_image]
-            self._image_file = open(imagename, "rb")
+            image_name = self._file_list[self._current_image]
+            self._image_file = open(image_name, "rb")
             try:
                 odb = displayio.OnDiskBitmap(self._image_file)
             except ValueError: