shell32/autocomplete: Revamp pwzsRegKeyPath handling so it can deal with arbitrary sizes and make it more robust.

Handle heap_alloc failure, reg strings without a \ character at all,
try harder to find the reg path (if only value fails the lookup), and
read the registry value with any size.
Signed-off-by: Gabriel Ivăncescu <gabrielopcode@gmail.com>
Signed-off-by: Huw Davies <huw@codeweavers.com>
Signed-off-by: Alexandre Julliard <julliard@winehq.org>