samples/kernel_event_logger: initialize variable

Fix usage of an uninitialized variable detected by Coverity.

In theory GCC should pick up this situation, but it does not. I've
experimented with adding -Wextra and -Wuninitialized but I cannot get
GCC to complain. I might be missing something else, but in the
meantime, this is a simple fix to remove this issue.

Change-Id: I6fec37719719dfaf7077ce1f464605c93efa8ea2
Coverity-ID: 152054
Signed-off-by: Inaky Perez-Gonzalez <inaky.perez-gonzalez@intel.com>