AudioSystem: Ensure no nullptr access

Brute force nullptr check for safety. A warning is
issued if nullptr is found, but it should not occur.

Flag: EXEMPT bugfix
Test: for run in {1..100}; do (sleep 8; echo $run; adb shell pkill audioserver); done
Bug: 379427790
Change-Id: I3ba7a89ecfaf35b658de9477e3f27dc3a59352b2
diff --git a/media/libaudioclient/AudioSystem.cpp b/media/libaudioclient/AudioSystem.cpp
index e3a96f5..8094d07 100644
--- a/media/libaudioclient/AudioSystem.cpp
+++ b/media/libaudioclient/AudioSystem.cpp
@@ -151,7 +151,11 @@
         {
             std::lock_guard l(mMutex);
             mValid = false;
-            mClient->clearIoCache();
+            if (mClient) {
+                mClient->clearIoCache();
+            } else {
+                ALOGW("%s: null client", __func__);
+            }
         }
         AudioSystem::reportError(DEAD_OBJECT);
     }
@@ -1004,7 +1008,11 @@
             mValid = false;
             client = mClient;
         }
-        client->onServiceDied();
+        if (client) {
+            client->onServiceDied();
+        } else {
+            ALOGW("%s: null client", __func__);
+        }
     }
 
     static constexpr mediautils::ServiceOptions options() {