Dead loop in plot_dot

**Describe the bug**
The following code will fall into dead loop and exausts all memory in system.

**To Reproduce**
```
import arviz
from numpy import nan

arviz.plot_dot([0.81449801, 0.8748649 ,        nan,        nan,        nan], show=False)
```

**Expected behavior**
Generate plot or report error, not fall into dead loop

**Additional context**
