osmo_io: Don't pretend to support backends without close_cb

Let's not pretend we support backends without a close_cb.  In such
situations nobody would actually close(2) the file descriptor,
but we would set iofd->fd to -1, effectively creating a file descriptor
leak.

Both of our two back-ends provide a close_cb, and we don't need to
consider hypothetical future back-ends that would not like to register
such a call-back.

Related: OS#6393
Change-Id: Id285f1d7b73ae5805aa618897016ae8b73bf892d
diff --git a/src/core/osmo_io.c b/src/core/osmo_io.c
index bcd4add..8b53aa6 100644
--- a/src/core/osmo_io.c
+++ b/src/core/osmo_io.c
@@ -787,8 +787,8 @@
 
 	iofd->pending = NULL;
 
-	if (osmo_iofd_ops.close)
-		rc = osmo_iofd_ops.close(iofd);
+	OSMO_ASSERT(osmo_iofd_ops.close);
+	rc = osmo_iofd_ops.close(iofd);
 	iofd->fd = -1;
 	return rc;
 }