AddCuePoint: fix memory leak on failure

quiets a static analysis warning

Change-Id: I288ab6216ecd066f7d6542a7465db1f182859c32
diff --git a/mkvmuxer/mkvmuxer.cc b/mkvmuxer/mkvmuxer.cc
index 9bf1924..7434d33 100644
--- a/mkvmuxer/mkvmuxer.cc
+++ b/mkvmuxer/mkvmuxer.cc
@@ -3386,8 +3386,10 @@
   cue->set_block_number(cluster->blocks_added());
   cue->set_cluster_pos(cluster->position_for_cues());
   cue->set_track(track);
-  if (!cues_.AddCue(cue))
+  if (!cues_.AddCue(cue)) {
+    delete cue;
     return false;
+  }
 
   new_cuepoint_ = false;
   return true;