fix(api): nvim_get_option_value does not clean up on FileType error #31219

Problem:
If there's an error in `FileType` autocmd, the filetype get-opt buffer
doesn't get cleaned up.

Solution:
Call `aucmd_restbuf`.

src/nvim/api/options.c

@@ -165,6 +165,14 @@ Object nvim_get_option_value(String name, Dict(option) *opts, Error *err)
 
   buf_T *ftbuf = do_ft_buf(filetype, &aco, err);
   if (ERROR_SET(err)) {
+    if (ftbuf != NULL) {
+      // restore curwin/curbuf and a few other things
+      aucmd_restbuf(&aco);
+
+      assert(curbuf != ftbuf);  // safety check
+      wipe_buffer(ftbuf, false);
+    }
+
     return (Object)OBJECT_INIT;
   }