MdeModulePkg/NullMemoryTest: Fix bug in CompatibleRangeTest

MdeModulePkg/NullMemoryTest: Fix bug in CompatibleRangeTest

CompatibleRangeTest() contains two bugs:
1. It doesn't reject the memory above 16MB
2. it cannot handle the case when the partial or whole range of
   requested memory is already tested.

The patch fixes the two bugs.

Contributed-under: TianoCore Contribution Agreement 1.1
Signed-off-by: Ruiyu Ni <ruiyu.ni@intel.com>
Reviewed-by: Michael D Kinney <michael.d.kinney@intel.com>
Reviewed-by: Liming Gao <liming.gao@intel.com>