[RFC] Fix memory leak in job_start().

Fix for memory leak in job_start().

If a new job cannot be started because no slots a free, we return early
without freeing the argv argument.
Not sure if this notation will be accepted by doxygen.
From what I know, doxygen allows a direction attribute with possible values in/out/in-out using just that style.
This is, `@param[in] ...`, etc.
So, I don't know if a value other than those allowed will be rejected.
It this in fact causes a problem, I'd just put `[consumed]` at the end of the line (for short descs), or on a new line (for longer descs).