Fix 48831, Step I: Mismatch issues bewteen ID icon and preview system.

- icon_id from ID and PreviewImage were not guaranteed to be in sync.
- PreviewImage one was not reset on file read.
- Through RNA e.g., it was possible to ensure an ID icon via its preview image,
  which was running code designed for custom previews/icons system, instead
  of generating correct 'auto ID icon'.

source/blender/blenloader/intern/readfile.c

@@ -2144,6 +2144,7 @@ static PreviewImage *direct_link_preview_image(FileData *fd, PreviewImage *old_p
 			}
 			prv->gputexture[i] = NULL;
 		}
+		prv->icon_id = 0;
 	}
 	
 	return prv;