chan_oss.c: Fix format ref leak in oss_read().

Change-Id: I0a5d56c7dcf327d60f86a4c25a23571733709fd0

channels/chan_oss.c

@@ -727,7 +727,7 @@ static struct ast_frame *oss_read(struct ast_channel *c)
 		return f;
 	/* ok we can build and deliver the frame to the caller */
 	f->frametype = AST_FRAME_VOICE;
-	f->subclass.format = ao2_bump(ast_format_slin);
+	f->subclass.format = ast_format_slin;
 	f->samples = FRAME_SIZE;
 	f->datalen = FRAME_SIZE * 2;
 	f->data.ptr = o->oss_read_buf + AST_FRIENDLY_OFFSET;