[rotorcraft] guidance_h speed ref saturation fix

Bound ref to max accel along reference vector.
If angle can't be computed, simply set both axes to max magnitude instead of zero.

Should (tm) fix #716
[rotorcraft] gh_max_speed|accel
Merge pull request #717 from paparazzi/fix_speed_ref_saturation

[rotorcraft] guidance_h speed ref saturation fix