Reproduction steps:
1. Open attached project (AlphaValue.zip)
2. Open "AlphaValue" scene
3. Select Image game object in Canvas
4. Hit play (Image Color alpha will be set to NaN by attached script)

Expected result:
Color alpha value bar of the Image is in its bounds

Actual result:
Color alpha value bar is out of bounds in negative direction (See image attached)

Same occurs if we open Color picker by clicking on that color.
Reproduces with materials too.

Reproduced on: Unity 5.2.5f1, 5.3.3f1, 5.4.1f1, 5.4.2p3, 5.5.0b11, 5.6.0a2