RTSPSource: check mHTTPService before dereferencing it.

Change-Id: Id92c5ed86742f9c5cb0a67207b26de61523fd009
diff --git a/media/libmediaplayerservice/nuplayer/RTSPSource.cpp b/media/libmediaplayerservice/nuplayer/RTSPSource.cpp
index af0351e..c861fd1 100644
--- a/media/libmediaplayerservice/nuplayer/RTSPSource.cpp
+++ b/media/libmediaplayerservice/nuplayer/RTSPSource.cpp
@@ -76,6 +76,11 @@
 }
 
 void NuPlayer::RTSPSource::prepareAsync() {
+    if (mIsSDP && mHTTPService == NULL) {
+        notifyPrepared(BAD_VALUE);
+        return;
+    }
+
     if (mLooper == NULL) {
         mLooper = new ALooper;
         mLooper->setName("rtsp");