Fix T53612: Blender crashes on CleanTracks with 'DELETE_SEGMENTS' and a disabled marker

Simple fix, which is totally safe for 2.79a!

source/blender/editors/space_clip/tracking_ops.c

@@ -1858,6 +1858,10 @@ static bool is_track_clean(MovieTrackingTrack *track, int frames, int del)
 		}
 	}
 
+	if (count == 0) {
+		ok = 0;
+	}
+
 	if (del) {
 		MEM_freeN(track->markers);