drop usage of sprintf

[grobian]

format.cpp:1606:16: warning: ‘int sprintf(char*, const char*, ...)’ is deprecated: This function is provided for compatibility reasons only.  Due to security concerns inherent in the design of sprintf(3), it is highly recommended that you use snprintf(3) instead. [-Wdeprecated-declarations]
 1606 |         sprintf(lb, "%s.vspace.after", item_name);
      |         ~~~~~~~^~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~

get rid of this