Fix NPE in resetVisibleLimits

* resetVisibleLimits can be called via BootReceiver, which isn't in the
  Email app process, so it can (and apparently did) get a RemoteException
* This causes the query to return null; we have to check for it or we
  get this NPE

Bug: 2564904
Change-Id: I4b75e3c74ac7d1276f609f2fc957afdaa8da2f64

src/com/android/email/Controller.java

@@ -488,7 +488,9 @@ public class Controller {
                         }
                     }
                 } finally {
-                    c.close();
+                    if (c != null) {
+                        c.close();
+                    }
                 }
             }
         }.start();