HTTPLiveSource: check for NULL before getTrackCount/Info

The effect is MediaPlayer returns a 0-length array when getTrackInfo is
called before PREPARED state.

Bug: 12029173
Change-Id: Ib3a48525eac07b04a2ff88ce199d66dcc61c1641
diff --git a/media/libstagefright/httplive/LiveSession.cpp b/media/libstagefright/httplive/LiveSession.cpp
index 8667a6b..7b18348 100644
--- a/media/libstagefright/httplive/LiveSession.cpp
+++ b/media/libstagefright/httplive/LiveSession.cpp
@@ -1019,11 +1019,19 @@
 }
 
 size_t LiveSession::getTrackCount() const {
-    return mPlaylist->getTrackCount();
+    if (mPlaylist == NULL) {
+        return 0;
+    } else {
+        return mPlaylist->getTrackCount();
+    }
 }
 
 sp<AMessage> LiveSession::getTrackInfo(size_t trackIndex) const {
-    return mPlaylist->getTrackInfo(trackIndex);
+    if (mPlaylist == NULL) {
+        return NULL;
+    } else {
+        return mPlaylist->getTrackInfo(trackIndex);
+    }
 }
 
 status_t LiveSession::selectTrack(size_t index, bool select) {