audio: fix audioSession null check

Test: make

Change-Id: Ied514f8fb7496143bad45504151ca16226c67767
diff --git a/services/audiopolicy/managerdefault/AudioPolicyManager.cpp b/services/audiopolicy/managerdefault/AudioPolicyManager.cpp
index eb61ec4..54c406c 100644
--- a/services/audiopolicy/managerdefault/AudioPolicyManager.cpp
+++ b/services/audiopolicy/managerdefault/AudioPolicyManager.cpp
@@ -1932,7 +1932,7 @@
     ALOG_ASSERT(inputDesc != 0);
 
     sp<AudioSession> audioSession = inputDesc->getAudioSession(session);
-    if (index < 0) {
+    if (audioSession == 0) {
         ALOGW("releaseInput() unknown session %d on input %d", session, input);
         return;
     }