basic: make check-pointer-deref clean

Add the needed assert changes to make the code clean
for the new check-pointer-deref script.

Note that for `_cmp()` functions its a bit of a question
if we want `assert()` or `POINTER_MAY_BE_NULL()` here. The
former is more correct but it adds a (small) runtime hit. If
we use POINTER_MAY_BE_NULL() it is compiled out but strictly
speaking it is wrong (so at least we would need a comment).

src/basic/log.c

@@ -950,6 +950,9 @@ int log_format_iovec(
                 const char *format,
                 va_list ap) {
 
+        assert(iovec);
+        assert(n);
+
         while (format && *n + 1 < iovec_len) {
                 va_list aq;
                 char *m;