Issue with drawing a constant line in a subplot

Please try the following code:
```
subplot(grid="2x1")

lines(1:10, repeat([1],10), panel=1)

lines(1:10, sin.(1:10), panel=2)

subplot(:show)
```
The code above fails in GMT 1.41.2. However, adding a ```limits``` option resolves the issue.
```
subplot(grid="2x1")

lines(1:10, repeat([1],10), panel=1, limits=(1,10,0,2))

lines(1:10, sin.(1:10), panel=2)

subplot(:show)
```

Reproduce the two snippets from the issue in GMT.jl 1.41.2, first with the constant line in subplot panel 1 and then with the explicit limits workaround. Start by tracing the subplot and lines entry points and compare how constant-valued data are handled; done means the first example runs without requiring limits.

Written by the indexing model from the issue text.