MdePkg BaseLib: Fix a corner case of Source and Destination overlap.

The overlap may happen when the address of Destination in
UnicodeStrToAsciiStr() or Source in AsciiStrToUnicodeStr()
is not two bytes aligned.

Contributed-under: TianoCore Contribution Agreement 1.0
Signed-off-by: Star Zeng <star.zeng@intel.com>
Reviewed-by: Liming Gao <liming.gao@intel.com>

git-svn-id: https://svn.code.sf.net/p/edk2/code/trunk/edk2@15665 6f19259b-4bc3-4df7-8a09-765794883524