ShellPkg/Dp: Add null pointer check
ShellPkg/Dp: Add null pointer check

Cc: Liming Gao <6480311aeeb4b006862f6d13ebabddc03f51e507@intel.com>
Cc: Hao Wu <f387b71200577e27ccf5abb090fda0ddd187ec6e@intel.com>
Contributed-under: TianoCore Contribution Agreement 1.1
Signed-off-by: Dandan Bi <1e8692d8ed8470b17e8ddd7612a0f029768dcb56@intel.com>
Reviewed-by: Liming Gao <6480311aeeb4b006862f6d13ebabddc03f51e507@intel.com>

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