Fix a bug when looping FFmpeg inputs that have no audio.

[nageru] / bmusb

diff --git a/bmusb b/bmusb
index 6a012a41c5422092cdac1f18a9019f37c0b85368..e85d7ee1c0541900c57dd505454289dfaab9e782 160000 (submodule)
--- a/bmusb
+++ b/bmusb
@@ -1 +1 @@
-Subproject commit 6a012a41c5422092cdac1f18a9019f37c0b85368
+Subproject commit e85d7ee1c0541900c57dd505454289dfaab9e782