yajl_gen: use strspn instead of open loop

[mathstuf]

---

As discussed in #21.

[mathstuf]

Ping?

[Jesin]

No need to use strlen which does a second iteration from the beginning of the string. You can just say:

if (indent[strspn(indent, "\n\v\f\t\r ")]) {
    g->indentString = NULL;
    rv = 0;
}