vttdemux,CloseFiles: check file pointer before closing

BUG=webm:1405

Change-Id: I7729277c632d40d478aa80ca6a009f95038196fb
diff --git a/vttdemux.cc b/vttdemux.cc
index c9abd5c..52a2661 100644
--- a/vttdemux.cc
+++ b/vttdemux.cc
@@ -523,8 +523,10 @@
     metadata_map_t::value_type& v = *i++;
     MetadataInfo& info = v.second;
 
-    fclose(info.file);
-    info.file = NULL;
+    if (info.file != NULL) {
+      fclose(info.file);
+      info.file = NULL;
+    }
   }
 }