ext_poisson does not build

### Checklist

- [X] I have searched for [similar issues](https://github.com/isl-org/Open3D/issues).
- [X] For Python issues, I have tested with the [latest development wheel](http://www.open3d.org/docs/latest/getting_started.html#development-version-pip).
- [X] I have checked the [release documentation](http://www.open3d.org/docs/release/) and the [latest documentation](http://www.open3d.org/docs/latest/) (for `master` branch).

### Describe the issue

In version 0.17.0, ext_poisson does not build because Werror=maybe_uninitialized and the compiler found that some variables may be uninitialized

### Steps to reproduce the bug

```python
followed http://www.open3d.org/docs/release/compilation.html
```

### Error message

/home/sven/git/Open3D/build/poisson/src/ext_poisson/PoissonRecon/Src/RegularTree.inl:251:36: error: ‘c’ may be used uninitialized [-Werror=maybe-uninitialized]
251 | if( !i || d>c ) c=d;

/home/sven/git/Open3D/build/poisson/src/ext_poisson/PoissonRecon/Src/RegularTree.inl: In function ‘int RegularTreeNode::maxDepth() const [with unsigned int Dim = 3; NodeData = FEMTreeNodeData; DepthAndOffsetType = short unsigned int]’:
/home/sven/git/Open3D/build/poisson/src/ext_poisson/PoissonRecon/Src/RegularTree.inl:247:21: note: ‘c’ was declared here
247 | int c , d;

/home/sven/git/Open3D/build/poisson/src/ext_poisson/PoissonRecon/Src/FEMTree.IsoSurface.specialized.inl:1562:37: error: ‘start’ may be used uninitialized [-Werror=maybe-uninitialized]
1562 | position[o] = Real( start + width*averageRoot );

/home/sven/git/Open3D/build/poisson/src/ext_poisson/PoissonRecon/Src/FEMTree.IsoSurface.specialized.inl: In lambda function:
/home/sven/git/Open3D/build/poisson/src/ext_poisson/PoissonRecon/Src/FEMTree.IsoSurface.specialized.inl:1511:22: note: ‘start’ was declared here
1511 | Real start , width;

### Expected behavior

everything compiles without warning or -Werror is not set :)

### Open3D, Python and System information

```markdown
- Operating system: Ubuntu 23.04
- Python version: 3.11.2 (main, Mar 13 2023, 12:18:29) [GCC 12.2.0]
- Open3D version: output from python: v0.17.0
- System architecture: x86
- Is this a remote workstation?: no
- How did you install Open3D?: build from source
- Compiler version (if built from source): gcc (Ubuntu 12.2.0-17ubuntu1) 12.2.0
```

### Additional information

_No response_