printk: check if oops log buffer's empty

Don't gather oops log buffer when it's empty.  This fixes infinite
/proc/kmsg print.

Change-Id: I2b6f7581eaa57f89f6e18a60b3e3e271b2b1ad0b
Signed-off-by: Joonwoo Park <joonwoop@codeaurora.org>