revert bug introduced in 1622607

PR #67 removed the assert that checked width != 0 in fn new_sub_renderer()
It seems commit 1622607 reintroduced the check, as a result some tables
again fail to render with TooNarrow error, although both the available space
seem enough and the visual appearance is satisfactory

src/render/text_renderer.rs

@@ -1137,9 +1137,6 @@ impl<D: TextDecorator> Renderer for SubRenderer<D> {
     }
 
     fn new_sub_renderer(&self, width: usize) -> crate::Result<Self> {
-        if width < 1 {
-            return Err(Error::TooNarrow);
-        }
         let mut result = SubRenderer::new(
             width,
             self.options.clone(),