of: Fix null pointer related warnings in base.c file

Fixes the following sparse warnings:
drivers/of/base.c:176:41: warning: Using plain integer as NULL pointer
drivers/of/base.c:178:37: warning: Using plain integer as NULL pointer
drivers/of/base.c:500:49: warning: Using plain integer as NULL pointer

Signed-off-by: Sachin Kamat <sachin.kamat@linaro.org>
Signed-off-by: Rob Herring <rob.herring@calxeda.com>