refactor: use xstrl{cpy,cat} on IObuff (#23648)

Replace usage of STR{CPY,CAT} with xstrl{cpy,cat} when using on IObuff

Co-authored-by: ii14 <ii14@users.noreply.github.com>

src/nvim/runtime.c

@@ -1547,7 +1547,7 @@ static inline char *add_dir(char *dest, const char *const dir, const size_t dir_
     xstrlcpy(IObuff, appname, appname_len + 1);
 #if defined(MSWIN)
     if (type == kXDGDataHome || type == kXDGStateHome) {
-      STRCAT(IObuff, "-data");
+      xstrlcat(IObuff, "-data", IOSIZE);
       appname_len += 5;
     }
 #endif