MediaDrm jni: get exception logs only if necessary

Bug: 162255728
Test: MediaDrmTest#testGetLogMessages
Change-Id: I39c203ff33dcc109120e9bbc1e089e6c8b7c90b3

media/jni/android_media_MediaDrm.cpp

@@ -424,7 +424,7 @@ static bool isSessionException(status_t err) {
 static bool throwExceptionAsNecessary(
         JNIEnv *env, const sp<IDrm> &drm, status_t err, const char *msg = NULL) {
     std::string msgStr;
-    if (drm != NULL) {
+    if (drm != NULL && err != OK) {
         msgStr = DrmUtils::GetExceptionMessage(err, msg, drm);
         msg = msgStr.c_str();
     }