CuePoint::Find: check Track pointer

replace assert with a check of the pointer, this is a public function so
should be tolerant of invalid parameters

BUG=webm:1415

Change-Id: I28a3a2ef905d62f11928dbbe54c119411d2b1f74
diff --git a/mkvparser/mkvparser.cc b/mkvparser/mkvparser.cc
index 6e1c26e..a3995de 100644
--- a/mkvparser/mkvparser.cc
+++ b/mkvparser/mkvparser.cc
@@ -2430,7 +2430,9 @@
 }
 
 const CuePoint::TrackPosition* CuePoint::Find(const Track* pTrack) const {
-  assert(pTrack);
+  if (pTrack == NULL) {
+    return NULL;
+  }
 
   const long long n = pTrack->GetNumber();