We should return after calling moveTaskToBack when there is no current tab.

Fix http://b/issue?id=2142160
diff --git a/src/com/android/browser/BrowserActivity.java b/src/com/android/browser/BrowserActivity.java
index e5434ed..eb79a3c 100644
--- a/src/com/android/browser/BrowserActivity.java
+++ b/src/com/android/browser/BrowserActivity.java
@@ -2100,6 +2100,7 @@
              * moveTaskToBack().
              */
             moveTaskToBack(true);
+            return;
         }
         WebView w = current.getWebView();
         if (w.canGoBack()) {