SourceLevelDebugPkg/DebugAgent: Add typecast to fix sign extension

SourceLevelDebugPkg/DebugAgent: Add typecast to fix sign extension

OffsetHigh is 16bit value and its type is UINT32 and defined in structure.
It will be 32bit int type after do 16-bit left-shift operation. Then it will
sign extension if cast it to UINT64 if its high bit is 1.

Contributed-under: TianoCore Contribution Agreement 1.0
Signed-off-by: Jeff Fan <jeff.fan@intel.com>
Reviewed-by: Ruiyu Ni <ruiyu.ni@intel.com>
Reviewed-by: Scott Duplichan <scott@notabs.org>
git-svn-id: https://svn.code.sf.net/p/edk2/code/trunk/edk2@17716 6f19259b-4bc3-4df7-8a09-765794883524