add safety check in case of short extensions

so eg if a two-letter or empty extension is used, `.txt` would break, this `max` call protects that.

modules/images.py

@@ -575,7 +575,7 @@ def save_image(image, path, basename, seed=None, prompt=None, extension='png', i
     fullfn_without_extension, extension = os.path.splitext(params.filename)
     if hasattr(os, 'statvfs'):
         max_name_len = os.statvfs(path).f_namemax
-        fullfn_without_extension = fullfn_without_extension[:max_name_len - len(extension)]
+        fullfn_without_extension = fullfn_without_extension[:max_name_len - max(4, len(extension))]
         params.filename = fullfn_without_extension + extension
     _atomically_save_image(image, fullfn_without_extension, extension)