Fix notice when setCover fails to retrieve the image

Signed-off-by: Kevin Decherf <kevin@kdecherf.com>

src/PHPePub/Core/EPub.php

@@ -1261,6 +1261,9 @@ function setCoverImage($fileName, $imageData = null, $mimetype = null) {
                 }
             }
             $image = ImageHelper::getImage($this, $fileName);
+            if (false === $image) {
+                return false;
+            }
             $imageData = $image['image'];
             $mimetype = $image['mime'];
             $fileName = preg_replace('#\.[^\.]+$#', "." . $image['ext'], $fileName);