Update epub.py

initialize covefile to None first

cps/epub.py

@@ -49,6 +49,7 @@ def get_epub_info(tmp_file_path, original_file_name, original_file_extension):
             epub_metadata[s] = "Unknown"
 
     coversection = tree.xpath("/pkg:package/pkg:manifest/pkg:item[@id='cover-image']/@href", namespaces=ns)
+    coverfile = None
     if len(coversection) > 0:
         coverfile = extractCover(zip, coversection[0], coverpath, tmp_file_path)
     else:
@@ -71,8 +72,6 @@ def get_epub_info(tmp_file_path, original_file_name, original_file_extension):
                 meta_cover_content = tree.xpath("/pkg:package/pkg:manifest/pkg:item[@id='"+meta_cover[0]+"']/@href",namespaces=ns)
                 if len(meta_cover_content) > 0:
                     coverfile = extractCover(zip, meta_cover_content[0], coverpath, tmp_file_path)
-            else:
-                coverfile = None
 
     if epub_metadata['title'] is None:
         title = original_file_name