kernel: print and reset grub_errno after each embedded config line

Otherwise it causes subsequent file open to fail, because grub_file_open
misinterprets set grub_errno for grub_file_get_device_name failure.

Closes: 46540

grub-core/kern/parser.c

@@ -268,6 +268,7 @@ grub_parser_execute (char *source)
       grub_parser_execute_getline (&line, 0, &source);
       grub_rescue_parse_line (line, grub_parser_execute_getline, &source);
       grub_free (line);
+      grub_print_error ();
     }
 
   return grub_errno;