SecurityPkg/SecureBootConfigDxe: Handle allocation failure gracefully

SecurityPkg/SecureBootConfigDxe: Handle allocation failure gracefully

The function AllocateCopyPool may return NULL, so need to do check
after calling it. This patch is to enhance the related logic.

Cc: Chao Zhang <chao.b.zhang@intel.com>
Cc: Qiu Shumin <shumin.qiu@intel.com>
Cc: Eric Dong <eric.dong@intel.com>
Contributed-under: TianoCore Contribution Agreement 1.0
Signed-off-by: Dandan Bi <dandan.bi@intel.com>
Reviewed-by: Chao Zhang <chao.b.zhang@intel.com>
Reviewed-by: Eric Dong <eric.dong@intel.com>