fix --ide-ast when there are errors (#13737)

# Description

This PR fixes #13732. However, I don't think it's a proper fix.
1. It doesn't really show what the problem is.
2. It kind of side-steps the error entirely.

I do think the change in span.rs may be valid because I don't think
span.end should ever be 0. In the example in 13732 the span end was
always 0 and so that made contains_span() return true, which seems like
a false positive.

The `checked_sub()` in ide.rs kind of just stops it from failing
outloud.

I'll leave it to smarter folks than me to land this if they think it's
worthy.

crates/nu-protocol/src/span.rs

@@ -130,7 +130,7 @@ impl Span {
     }
 
     pub fn contains_span(&self, span: Self) -> bool {
-        self.start <= span.start && span.end <= self.end
+        self.start <= span.start && span.end <= self.end && span.end != 0
     }
 
     /// Point to the space just past this span, useful for missing values

src/ide.rs

@@ -642,8 +642,8 @@ pub fn ast(engine_state: &mut EngineState, file_path: &str) {
                 {
                     "type": "ast",
                     "span": {
-                        "start": span.start - offset,
-                        "end": span.end - offset,
+                        "start": span.start.checked_sub(offset),
+                        "end": span.end.checked_sub(offset),
                     },
                     "shape": shape.to_string(),
                     "content": content // may not be necessary, but helpful for debugging