maple_tree: fix warning comparing pointer to 0

Avoid pointer type value compared with 0 to make code clear.

./tools/testing/radix-tree/maple.c:34142:15-16: WARNING comparing pointer to 0.

Link: https://lkml.kernel.org/r/20231208020450.7003-1-jiapeng.chong@linux.alibaba.com


Reported-by: Abaci Robot <abaci@linux.alibaba.com>
Closes: https://bugzilla.openanolis.cn/show_bug.cgi?id=7696


Signed-off-by: Jiapeng Chong <jiapeng.chong@linux.alibaba.com>
Cc: Liam R. Howlett <Liam.Howlett@oracle.com>
Cc: Matthew Wilcox (Oracle) <willy@infradead.org>
Signed-off-by: Andrew Morton <akpm@linux-foundation.org>