SecurityPkg Tcg2Dxe: ASSERT to ensure 'VarData' is not NULL

The logic in functions ReadAndMeasureVariable() and MeasureVariable()
within Tcg2Dxe ensure that 'VarData' will not be NULL before calling
TcgDxeHashLogExtendEvent() at line 1716.

This commit adds ASSERT as warnings for the case that will not happen.

Cc: Jiewen Yao <jiewen.yao@intel.com>
Cc: Chao Zhang <chao.b.zhang@intel.com>
Contributed-under: TianoCore Contribution Agreement 1.0
Signed-off-by: Hao Wu <hao.a.wu@intel.com>
Reviewed-by: Jiewen Yao <jiewen.yao@intel.com>
Reviewed-by: Chao Zhang <chao.b.zhang@intel.com>