log: make each string generated in log_format_iovec() NUL terminated

Nowadays, we append an extra NUL for each data if possible for safety.
We already do the same for example at write_to_kmsg(), log_do_context(),
write_to_journal(), log_struct_iovec_internal(), and so on.
This does not change any behavior, as the iov_len field is unchanged.

src/basic/log.c

@@ -950,8 +950,6 @@ int log_format_iovec(
                 const char *format,
                 va_list ap) {
 
-        static const char nl = '\n';
-
         while (format && *n + 1 < iovec_len) {
                 va_list aq;
                 char *m;
@@ -975,7 +973,7 @@ int log_format_iovec(
 
                 iovec[(*n)++] = IOVEC_MAKE_STRING(m);
                 if (newline_separator)
-                        iovec[(*n)++] = IOVEC_MAKE((char *)&nl, 1);
+                        iovec[(*n)++] = IOVEC_MAKE_STRING("\n");
 
                 format = va_arg(ap, char *);
         }