of: fix uninitialized variable warning for overlay test

gcc warns that an empty device tree would cause undefined behavior:

drivers/of/unittest.c: In function 'of_unittest':
drivers/of/unittest.c:2199:25: warning: 'last_sibling' may be used uninitialized in this function [-Wmaybe-uninitialized]

This adds an initialization of the variable to zero, which we handle
correctly.

Fixes: 81d0848f ("of: Add unit tests for applying overlays")
Signed-off-by: Arnd Bergmann <arnd@arndb.de>
Signed-off-by: Rob Herring <robh@kernel.org>