Fix a null dereference

Change-Id: I3df446b90c1607782778749de7ba0f8c00698c33
diff --git a/services/surfaceflinger/DisplayHardware/HWComposer.cpp b/services/surfaceflinger/DisplayHardware/HWComposer.cpp
index ff887e4..4af274b 100644
--- a/services/surfaceflinger/DisplayHardware/HWComposer.cpp
+++ b/services/surfaceflinger/DisplayHardware/HWComposer.cpp
@@ -84,7 +84,9 @@
 
 status_t HWComposer::commit() const {
     int err = mHwc->set(mHwc, mDpy, mSur, mList);
-    mList->flags &= ~HWC_GEOMETRY_CHANGED;
+    if (mList) {
+        mList->flags &= ~HWC_GEOMETRY_CHANGED;
+    }
     return (status_t)err;
 }