staging/rdma/hfi1: Fix a possible null pointer dereference

A code inspection pointed out that kmalloc_array may return NULL and
memset doesn't check the input pointer for NULL, resulting in a possible
NULL dereference. This patch fixes this.

Reviewed-by: Mike Marciniszyn <mike.marciniszyn@intel.com>
Signed-off-by: Easwar Hariharan <easwar.hariharan@intel.com>
Signed-off-by: Greg Kroah-Hartman <gregkh@linuxfoundation.org>