Fix panic in split_imports assist

The fix is admittedly quit literally just papering over.

Long-term, I see two more principled approaches:

* we switch to a fully tree-based impl, without parse . to_string
  step; with this approach, there shouldn't be any panics. The results
  might be nonsensical, but so was the original input.

* we preserve the invariant that re-parsing constructed node is an
  identity, and make all the `make_xxx` method return an `Option`.

closes #4044

crates/ra_syntax/src/algo.rs

@@ -10,8 +10,8 @@ use ra_text_edit::TextEditBuilder;
 use rustc_hash::FxHashMap;
 
 use crate::{
-    AstNode, Direction, NodeOrToken, SyntaxElement, SyntaxNode, SyntaxNodePtr, SyntaxToken,
-    TextRange, TextUnit,
+    AstNode, Direction, NodeOrToken, SyntaxElement, SyntaxKind, SyntaxNode, SyntaxNodePtr,
+    SyntaxToken, TextRange, TextUnit,
 };
 
 /// Returns ancestors of the node at the offset, sorted by length. This should
@@ -90,6 +90,10 @@ pub fn neighbor<T: AstNode>(me: &T, direction: Direction) -> Option<T> {
     me.syntax().siblings(direction).skip(1).find_map(T::cast)
 }
 
+pub fn has_errors(node: &SyntaxNode) -> bool {
+    node.children().any(|it| it.kind() == SyntaxKind::ERROR)
+}
+
 #[derive(Debug, PartialEq, Eq, Clone, Copy)]
 pub enum InsertPosition<T> {
     First,