Check for valid coords before getting word at coords

Fixes #1287

src/SelectionManager.ts

@@ -634,6 +634,11 @@ export class SelectionManager extends EventEmitter implements ISelectionManager
    * @param coords The coordinates to get the word at.
    */
   private _getWordAt(coords: [number, number], allowWhitespaceOnlySelection: boolean): IWordPosition {
+    // Ensure coords are within viewport (eg. not within scroll bar)
+    if (coords[0] >= this._terminal.cols) {
+      return null;
+    }
+
     const bufferLine = this._buffer.lines.get(coords[1]);
     if (!bufferLine) {
       return null;