Steps to repro:

1. Open attached project;
2. In the Project folder go to Assets -> Scenes -> API;
3. Open 'СreateTerrainFromAPIbug' scene;
4. Launch the scene.

Actual result:
Editor hangs and errors are spammed in the console:

Invalid AABB aabb
UnityEditor.DockArea:OnGUI()

See attached gif video.

The bug is caused by passing zero variables to terrainData.size.

Expected result:
Message "Terrain size should be bigger than 1" should be displayed. The behaviour should be consistent with setting terrain size in Terrain Component's inspector (TerrainSettings), where values lower then 1 are always clamped to 1.

Notes:
- Reproducible in Unity 5.5.0b4, 5.4.0f3, 5.3.6p1;
- Reproducible on OSX 10.11.6;
- Partial regression from 5.4 on OSX (Editor doesn't hang on this platform in 5.3.6p1).