Fix a crash when the stop might be called due to some error before start in RTSPSource

o related-to-bug: 7507224

Change-Id: Ic8bfec13097b824ba337a01c9b00c98af2a33f43
diff --git a/media/libmediaplayerservice/nuplayer/RTSPSource.cpp b/media/libmediaplayerservice/nuplayer/RTSPSource.cpp
index 5a7a785..6df2ddd 100644
--- a/media/libmediaplayerservice/nuplayer/RTSPSource.cpp
+++ b/media/libmediaplayerservice/nuplayer/RTSPSource.cpp
@@ -57,9 +57,7 @@
 }
 
 NuPlayer::RTSPSource::~RTSPSource() {
-    if (mLooper != NULL) {
-        mLooper->stop();
-    }
+   mLooper->stop();
 }
 
 void NuPlayer::RTSPSource::start() {
@@ -86,6 +84,9 @@
 }
 
 void NuPlayer::RTSPSource::stop() {
+    if (mLooper == NULL) {
+        return;
+    }
     sp<AMessage> msg = new AMessage(kWhatDisconnect, mReflector->id());
 
     sp<AMessage> dummy;