Create file after the image was retrieved

`post.get_image_data()` can throw, creating file and later calling the function can create empty "image" files, confusing the program.

scraper/server/__main__.py

@@ -124,8 +124,9 @@ def get_image(post_id: int = None):
         return send_file(path)
     else:
         post = py34.Post(entry.post, entry.dir, entry.image, entry.tags, entry.thumbnail)
+        image = post.get_image_data()
         with open(path, "wb") as file:
-            file.write(post.get_image_data())
+            file.write(image)
         return send_file(path)