Fix "ordered comparison between pointer and zero".

From Clang 4.x releases, DR583 and DR1512 will be addressed.
This patch, in advance, fixes the error(s).

Test: `mmma bootable/recovery`
Change-Id: I29dc85ae681307c322ab3a698c3f3bbad1c784ee
Signed-off-by: MinSeong Kim <minseong.kim@linaro.org>