Update xterm typings

src/typings/xterm.d.ts

@@ -950,7 +950,7 @@ declare module 'xterm' {
 		 *
 		 * @param x The character index to get.
 		 */
-		getCell(x: number): IBufferCell;
+		getCell(x: number): IBufferCell | undefined;
 
 		/**
 		 * Gets the line as a string. Note that this is gets only the string for the line, not taking
@@ -982,6 +982,7 @@ declare module 'xterm' {
 
 
 
+
 // Modifications to official .d.ts below
 declare module 'xterm' {
 	interface TerminalCore {