ffmpeg: fix boolean inversion in ffmpeg_tag()

ffmpeg_tag() deleted the tag when ffmpeg_helper() returned success.
The return value was interpreted incorrectly, it should return the tag
on success.

src/decoder/ffmpeg_plugin.c

@@ -338,7 +338,7 @@ static struct tag *ffmpeg_tag(const char *file)
 	base.tag = tag_new();
 
 	ret = ffmpeg_helper(&input, ffmpeg_tag_internal, &base);
-	if (ret) {
+	if (!ret) {
 		tag_free(base.tag);
 		base.tag = NULL;
 	}