Blender Git Loki

Fix T67671: Blender crashes when resizing sidebar with a colorbamp

The amount of triangles drawn by the button depends on its `sizex`.
See:
```
immBegin(GPU_PRIM_TRI_STRIP, (sizex + 1) * 2);
```
Therefore the `sizex` cannot be 0.

Solution a bit similar to rB56b0cd1d.

Ref T67671

Reviewers: fclem, brecht

Maniphest Tasks: T67671

Differential Revision: https://developer.blender.org/D5347