dbgk: kdbg: Attempt to fix Coverity defects

Coverity defects fixes :

• Possibility of fixing CID 716571: Resource leak (but I'm not sure) in ntoskrnl/dbgk/dbgkobj.c : ExFreePool for DebugEvents --> ExFreePoolWithTag (with the good tag )
• Attempt to resolve CID 515128 & 515129: Unitialized scalar variable for Gdtr.Base and Idtr.Base in ntoskrnl/kdbg/kdb_cli.c, by initializing the structures to their default value : KDESCRIPTOR blah = {};
• Fix CID 500432: Dereference after null check in ntoskrnl/kdbg/i386/i386-dis.c and ntoskrnl/kdbg/amd64/i386-dis.c, by putting a 'if (pointer != NULL)' guard. Remark: indentation sucks a lot in these two files.