ShellPkg: add array index check for shell delay option

ShellPkg: add array index check for shell delay option

BZ: https://bugzilla.tianocore.org/show_bug.cgi?id=1528

Shell delay option without parameters do not check the
index of shell parameter argv. Add index check to avoid
invalid pointer references.

Contributed-under: TianoCore Contribution Agreement 1.1
Signed-off-by: Zhichao Gao <zhichao.gao@intel.com>
Cc: Liming Gao <liming.gao@intel.com>
Cc: Ray Ni <ray.ni@intel.com>
Reviewed-by: Jaben Carsey <jaben.carsey@intel.com>
Reviewed-by: Liming Gao <liming.gao@intel.com>