blktrace: delete garbage for message trace

commit f4a1d08c introduces a regression. Originally for
BLK_TN_MESSAGE, we add message in trace and return. The commit ignores
the early return and add garbage info.

Signed-off-by: Shaohua Li <shli@fb.com>
Reviewed-by: Jeff Moyer <jmoyer@redhat.com>
Signed-off-by: Jens Axboe <axboe@fb.com>