strbuf_addstr_safe
static int strbuf_addstr_safe(struct strbuf *sb, const char *s);
strbuf_addstr_safe(&err_output, buf);
strbuf_addstr_safe(&err_output, buf);
strbuf_addstr_safe(&merged, child_test->err_output.buf);
strbuf_addstr_safe(&merged, err_output.buf);
strbuf_addstr_safe(&child->err_output, buf);