ACPICA: Fix for access to deleted object <regression>

Fixes problem introduced in 20080123, with fix for Unload operator.
Parse tree object can be already deleted; must use the opcode
within the WalkState.

ACPI: kmemcheck: Caught 16-bit read from freed memory
http://bugzilla.kernel.org/show_bug.cgi?id=10669

Signed-off-by: Lin Ming <ming.m.lin@intel.com>
Signed-off-by: Bob Moore <robert.moore@intel.com>
Signed-off-by: Len Brown <len.brown@intel.com>