reject negative inv.a

Turns out we _can_ create a (only very very slightly) negative a term
when inverting a profile that passes our sanity tests.

Reject them like we do in fitting.  Was thinking about nudging it up to
0, but that would imply some strange things, like x not mattering...

Bug: oss-fuzz:16581
Change-Id: I2b30a36250936042da08e0f1771f5dc7610b9853
Reviewed-by: Brian Osman <>
Commit-Queue: Mike Klein <>
4 files changed