SF: check for emptiness before accessing fence ring buffer

Fixes: 356476001
Change-Id: Ib93090ea7dcbb97dcb1c823e4f3a4f10d081153c
Test: presubmit
Flag: EXEMPT bugfix
diff --git a/services/surfaceflinger/Scheduler/src/FrameTargeter.cpp b/services/surfaceflinger/Scheduler/src/FrameTargeter.cpp
index 8adf2a6..3ee1e54 100644
--- a/services/surfaceflinger/Scheduler/src/FrameTargeter.cpp
+++ b/services/surfaceflinger/Scheduler/src/FrameTargeter.cpp
@@ -69,7 +69,10 @@
 
 const FenceTimePtr& FrameTarget::presentFenceForPreviousFrame() const {
     if (FlagManager::getInstance().allow_n_vsyncs_in_targeter()) {
-        return mPresentFences.back().fenceTime;
+        if (mPresentFences.size() > 0) {
+            return mPresentFences.back().fenceTime;
+        }
+        return FenceTime::NO_FENCE;
     }
 
     return mPresentFencesLegacy.front().fenceTime;