Fix NPE during dumpsys

When running dumpsys SurfaceFlinger during boot, it's possible that
mFlagManager is still null.

Bug: 203165976
Test: adb shell dumpsys SurfaceFlinger
Change-Id: Idff6d5e1267591813e960490539e1a5aaaea0163
diff --git a/services/surfaceflinger/SurfaceFlinger.cpp b/services/surfaceflinger/SurfaceFlinger.cpp
index 425b78b..7164467 100644
--- a/services/surfaceflinger/SurfaceFlinger.cpp
+++ b/services/surfaceflinger/SurfaceFlinger.cpp
@@ -5436,7 +5436,9 @@
     /*
      * Dump flag/property manager state
      */
-    mFlagManager->dump(result);
+    if (mFlagManager != nullptr) {
+        mFlagManager->dump(result);
+    }
 
     result.append(mTimeStats->miniDump());
     result.append("\n");