selftests/resctrl: Return NULL if malloc_and_init_memory() did not alloc mem

malloc_and_init_memory() in fill_buf isn't checking if memalign()
successfully allocated memory or not before accessing the memory.

Check the return value of memalign() and return NULL if allocating
aligned memory fails.

Fixes: a2561b12 ("selftests/resctrl: Add built in benchmark")
Co-developed-by: Fenghua Yu <fenghua.yu@intel.com>
Signed-off-by: Fenghua Yu <fenghua.yu@intel.com>
Signed-off-by: Ilpo Järvinen <ilpo.jarvinen@linux.intel.com>
Reviewed-by: Reinette Chatre <reinette.chatre@intel.com>
Signed-off-by: Shuah Khan <skhan@linuxfoundation.org>