wrong memory access in i2o_block_device_lock()

This patch fixes access to memory that has not been allocated:
i2o_msg_get_wait() can returns errors different from I2O_QUEUE_EMPTY.  But the
result is checked only against this code.  If it is not I2O_QUEUE_EMPTY then
we dereference the error code as the pointer later.

Signed-off-by: Vasily Averin <vvs@sw.ru>
Acked-by: Alan Cox <alan@lxorguk.ukuu.org.uk>
Cc: Markus Lidel <Markus.Lidel@shadowconnect.com>
Acked-by: Kirill Korotaev <dev@openvz.org>
Signed-off-by: Andrew Morton <akpm@linux-foundation.org>
Signed-off-by: Linus Torvalds <torvalds@linux-foundation.org>