To reproduce:
1. Open user's project, in Project window click Unlit_NegativeAlpha material
2. In Inspector window, right-click material, select Edit Shader
3. In opened shader's file go to line 39 and make sure alpha value is negative, feel free to change it up to different negative values

Expected: shader's alpha value is clamped at 0 - no matter the negative value, material should remain invisible
Actual: shader's alpha value is not restricted to any negative float, material becomes black, has interesting shading effects

Reproduced in: 5.6.1p2, 5.6.1p3, 2017.1.0b9, 2017.2.0a3
Not reproduced in: 5.4.5p3, 5.5.4f1
Regression starts in: 5.6.0a1

Note: check picture attached for comparison of correct and incorrect shading