libgui: disallow NULL Fence pointers

This change eliminates the uses of a NULL sp<Fence> indicating that no waiting
is required.  Instead we use a non-NULL but invalid Fence object for which the
wait methods will return immediately.

Bug: 7892871
Change-Id: I5360aebe3090422ef6920d56c99fc4eedc642e48
diff --git a/services/surfaceflinger/SurfaceFlingerConsumer.cpp b/services/surfaceflinger/SurfaceFlingerConsumer.cpp
index dc9089e..e427072 100644
--- a/services/surfaceflinger/SurfaceFlingerConsumer.cpp
+++ b/services/surfaceflinger/SurfaceFlingerConsumer.cpp
@@ -15,6 +15,7 @@
  */
 
 #define ATRACE_TAG ATRACE_TAG_GRAPHICS
+//#define LOG_NDEBUG 0
 
 #include "SurfaceFlingerConsumer.h"