Fix crash when finalizing indexing of video tracks with no parser

The Matroska indexer doesn't keep the CodecContext around when it won't
be used for anything due to lavc not having a parser for the track's
video format, so don't assume it exists in Finalize().

Closes #166.

src/core/indexing.cpp

@@ -131,7 +131,7 @@ void FFMS_Index::Finalize(std::vector<SharedVideoContext> const& video_contexts)
 
 		if (track.TT != FFMS_TYPE_VIDEO) continue;
 
-		if (video_contexts[i].CodecContext->has_b_frames) {
+		if (video_contexts[i].CodecContext && video_contexts[i].CodecContext->has_b_frames) {
 			track.MaxBFrames = video_contexts[i].CodecContext->has_b_frames;
 			continue;
 		}