IntelFrameworkModulePkg/LegacyBootMaintUiLib: Add NULL pointer check

IntelFrameworkModulePkg/LegacyBootMaintUiLib: Add NULL pointer check

mLegacyBootOptionPrivate pointer is initialized in Constructor function
with if condition check, but it's used in Destructor function directly
without any check. Now add the NULL pointer check.

Cc: Eric Dong <eric.dong@intel.com>
Cc: Liming Gao <liming.gao@intel.com>
Contributed-under: TianoCore Contribution Agreement 1.0
Signed-off-by: Dandan Bi <dandan.bi@intel.com>
Reviewed-by: Eric Dong <eric.dong@intel.com>