Fix panic from indenting on tree with errors

`deepest_preceding` is known to be a descendant of `node`. Repeated
calls of `Node::parent` _should_ eventually turn `deepest_preceding`
into `node`, but when the node is errored (the tree contains a syntax
error), `Node::parent` returns None.

In the typescript case:

    if(true) &&true
    //      ^ press enter here

The tree is:

    (program [0, 0] - [1, 0]
      (if_statement [0, 0] - [0, 15]
        condition: (parenthesized_expression [0, 2] - [0, 8]
          (true [0, 3] - [0, 7]))
        consequence: (expression_statement [0, 8] - [0, 15]
          (binary_expression [0, 8] - [0, 15]
            left: (identifier [0, 8] - [0, 8])
            right: (true [0, 11] - [0, 15])))))

`node` is the `program` node and `deepest_preceding` is the
`binary_expression`. The tree is errored on the `binary_expression`
node with `(MISSING identifier [0, 8] - [0, 8])`.

In the C++ case:

    ; <<
    // press enter after the ';'

The tree is:

    (translation_unit [0, 0] - [1, 0]
      (expression_statement [0, 0] - [0, 1])
      (ERROR [0, 1] - [0, 4]
        (identifier [0, 1] - [0, 1])))

`node` is the `translation_unit` and `deepest_preceding` is the `ERROR`
node.

In both cases, `Node::parent` on the errored node returns None.

helix-core/src/indent.rs

@@ -511,8 +511,12 @@ fn extend_nodes<'a>(
             *node = deepest_preceding;
             break;
         }
-        // This parent always exists since node is an ancestor of deepest_preceding
+        // If the tree contains a syntax error, `deepest_preceding` may not
-        deepest_preceding = deepest_preceding.parent().unwrap();
+        // have a parent despite being a descendant of `node`.
+        deepest_preceding = match deepest_preceding.parent() {
+            Some(parent) => parent,
+            None => return,
+        }
     }
 }