vttdemux::ChapterAtomParser: check for NULL display string

prevents segfault due to strlen(NULL)

BUG=webm:1382

Change-Id: I536663e287d151e11bf7074349a34d922cb9856a
diff --git a/vttdemux.cc b/vttdemux.cc
index b09a9b3..c9abd5c 100644
--- a/vttdemux.cc
+++ b/vttdemux.cc
@@ -262,6 +262,8 @@
     const mkvparser::Chapters::Display* display)
     : display_(display) {
   str_ = display->GetString();
+  if (str_ == NULL)
+    return;
   const size_t len = strlen(str_);
   str_end_ = str_ + len;
 }
@@ -269,7 +271,7 @@
 ChapterAtomParser::~ChapterAtomParser() {}
 
 int ChapterAtomParser::GetChar(char* c) {
-  if (str_ >= str_end_)  // end-of-stream
+  if (str_ == NULL || str_ >= str_end_)  // end-of-stream
     return 1;  // per the semantics of libwebvtt::Reader::GetChar
 
   *c = *str_++;  // consume this character in the stream