MdeModulePkg: Fix bug of AllocatePages for large alignment

MdeModulePkg: Fix bug of AllocatePages for large alignment

DescEnd will be clipped for alignment in CoreFindFreePagesI, and it
may fall below DescStart, when alignment is more than 16KB (included)
and both DescStart and original DescEnd fall into a single range of
such alignment. This results in a huge size (Negative number in
unsigned type) for this descriptor, fulfilling the allocation
requirement but failing to run ConvertPages; at last it causes
occasional failure of AllocatePages.

A simple comparison is added to ensure we would never get a negative
number.

Contributed-under: TianoCore Contribution Agreement 1.0
Signed-off-by: Heyi Guo <heyi.guo@linaro.org>
Reviewed-by: Liming Gao <liming.gao@intel.com>
Acked-by: Ard Biesheuvel <ard.biesheuvel@linaro.org>
git-svn-id: https://svn.code.sf.net/p/edk2/code/trunk/edk2@17575 6f19259b-4bc3-4df7-8a09-765794883524