Here's the FIP equation:
This year's FIP constant is
3.05
Checking the math: (13*0HR + 3*(0BB+0HBP) - 2*(32K))/16IP + 3.05 = -0.95
If Jansen had given up 1 HR his FIP would still be negative, but only -0.14. If he gives up 1 HR and 1BB, his FIP is 0.05.
TLDR - it's REAAALY hard to maintain a negative FIP all season. If Jansen kept pitching an 18 K/9 with 0 walks, for ROS, he could only give up 4 HRs total and still maintain a negative FIP. He only gave up 4 HR last year... so I think it's actually doable. Crazy stuff