Fix bogus bracket highlighting being generated at end of document

FIX: Fix a crash in `bracketMatching` when composing at end of document.

Closes codemirror/dev#1687

Author: marijnh

src/matchbrackets.ts

@@ -187,6 +187,7 @@ function matchMarkedBrackets(_state: EditorState, _pos: number, dir: -1 | 1, tok
 
 function matchPlainBrackets(state: EditorState, pos: number, dir: number, tree: Tree,
                             tokenType: NodeType, maxScanDistance: number, brackets: string) {
+  if (dir < 0 ? !pos : pos == state.doc.length) return null
   let startCh = dir < 0 ? state.sliceDoc(pos - 1, pos) : state.sliceDoc(pos, pos + 1)
   let bracket = brackets.indexOf(startCh)
   if (bracket < 0 || (bracket % 2 == 0) != (dir > 0)) return null