This changes the GlobalUnlock todo_wine as we previously considered the
invalid mem handle as a valid pointer, and we now instead check handle
validity.

Returning TRUE in for an invalid handle doesn't seem very important, and
a corner case, and it breaks compatibility and imm32 tests.
Signed-off-by: Rémi Bernon <rbernon@codeweavers.com>
Signed-off-by: Alexandre Julliard <julliard@winehq.org>