lavf: fall back to a more meaningful log instance name

An uninitialized AVFormatContext instance with neither iformat nor
oformat set will currently log as 'NULL', which is confusing and
unhelpful. Print 'AVFormatContext' instead, which provides more
information.

This happens e.g. if choosing an output format fails in
avformat_alloc_output_context2().

E.g. with the following commandline:
 ffmpeg -i <input> -f foobar -y /dev/null
before: [NULL @ 0x5580377834c0] Requested output format 'foobar' is not a suitable output format
after:  [AVFormatContext @ 0x55fa15bb34c0] Requested output format 'foobar' is not a suitable output format

libavformat/options.c

@@ -44,7 +44,7 @@ static const char* format_to_name(void* ptr)
     AVFormatContext* fc = (AVFormatContext*) ptr;
     if(fc->iformat) return fc->iformat->name;
     else if(fc->oformat) return fc->oformat->name;
-    else return "NULL";
+    else return fc->av_class->class_name;
 }
 
 static void *format_child_next(void *obj, void *prev)