test_map: initialize memory block

This is not a strictly necessary initialization, as the data is not
used, but will keep Coverity happy. It being a testcase, there are no
size or speed penalties to the overall kernel and avoids having to
manage a whitelist for an issue in scanning tools.

Change-Id: I0ddcf43ca1114356d58f93de57232864246ffe07
Coverity-ID: 152052
Signed-off-by: Inaky Perez-Gonzalez <inaky.perez-gonzalez@intel.com>