Fix crash when destroying a DuplicatingThread

Bug: 8121343
Change-Id: I9b1fc56c753e465e8a08916022e52db6c3a36906
diff --git a/services/audioflinger/AudioFlinger.cpp b/services/audioflinger/AudioFlinger.cpp
index 52fa576..1270825 100644
--- a/services/audioflinger/AudioFlinger.cpp
+++ b/services/audioflinger/AudioFlinger.cpp
@@ -374,6 +374,9 @@
 
 void AudioFlinger::unregisterWriter(const sp<NBLog::Writer>& writer)
 {
+    if (writer == 0) {
+        return;
+    }
     sp<IMemory> iMemory(writer->getIMemory());
     if (iMemory == 0) {
         return;