In the latest surprise from fallen favorite Zenefits, CEO David Sacks has given Series C investors a bigger stake in the company to compensate them for its shrinking valuation of just $2 billion — less than half what it was just over a year ago. Investors put $500 million into the then-high-flying health care insurance and benefits startup last May, which at the time gave them an 11 percent stake in a venture valued at $4.5 billion. In return, Zenefits is requiring that those investors agree…