SurfaceFlinger: Fix null pointer exception

Fixes a null pointer exception in getActiveConfig

Bug: 19416124
Change-Id: I45a01f67affb63a348866ce42a6013c5eb71fe8f
diff --git a/services/surfaceflinger/SurfaceFlinger.cpp b/services/surfaceflinger/SurfaceFlinger.cpp
index 6a5a39e..1069580 100644
--- a/services/surfaceflinger/SurfaceFlinger.cpp
+++ b/services/surfaceflinger/SurfaceFlinger.cpp
@@ -617,7 +617,11 @@
 }
 
 int SurfaceFlinger::getActiveConfig(const sp<IBinder>& display) {
-    return getDisplayDevice(display)->getActiveConfig();
+    sp<DisplayDevice> device(getDisplayDevice(display));
+    if (device != NULL) {
+        return device->getActiveConfig();
+    }
+    return BAD_VALUE;
 }
 
 void SurfaceFlinger::setActiveConfigInternal(const sp<DisplayDevice>& hw, int mode) {