Fix bug where file extension had an extra '.' under some circumstances

Fix bug where under some circumstances an extra "." was inserted between the file base name and the file extension.
The bug is triggered when the extension argument is one of "jpg", "jpeg", or "webp", and the image exceeds the format's dimension limit. Then the extension variable is set to ".png", resulting in the fullfn variable to evaluate to a string ending with "..png".

modules/images.py

@@ -653,7 +653,7 @@ def save_image(image, path, basename, seed=None, prompt=None, extension='png', i
     # WebP and JPG formats have maximum dimension limits of 16383 and 65535 respectively. switch to PNG which has a much higher limit
     if (image.height > 65535 or image.width > 65535) and extension.lower() in ("jpg", "jpeg") or (image.height > 16383 or image.width > 16383) and extension.lower() == "webp":
         print('Image dimensions too large; saving as PNG')
-        extension = ".png"
+        extension = "png"
 
     if save_to_dirs is None:
         save_to_dirs = (grid and opts.grid_save_to_dirs) or (not grid and opts.save_to_dirs and not no_prompt)