MdeModulePkg/RegularExpressionDxe: Add two missing null pointer checks
After update Oniguruma from v6.9.0 to v6.9.3, two null pointer check
should be added.
This change bases on the patch https://edk2.groups.io/g/devel/message/45183.
Cc: Jian J Wang <jian.j.wang@intel.com> Cc: Hao A Wu <hao.a.wu@intel.com> Signed-off-by: Liming Gao <liming.gao@intel.com> Reviewed-by: Hao A Wu <hao.a.wu@intel.com>