fix(completion): avoid invalid position

Author: iamcco

.gitignore

@@ -63,3 +63,4 @@ typings/
 .env
 
 out/
+.DS_Store

package.json

@@ -1,6 +1,6 @@
 {
   "name": "vim-language-server",
-  "version": "2.2.10",
+  "version": "2.2.11",
   "description": "vim language server",
   "keywords": [
     "viml",

src/common/util.ts

@@ -131,6 +131,11 @@ export function getWordFromPosition(
     return;
   }
 
+  // invalid character which less than 0
+  if (position.character < 0) {
+    return
+  }
+
   const character = doc.getText(
     Range.create(
       Position.create(position.line, position.character),