Btrfs: add NULL check for do_walk_down()
Btrfs: add NULL check for do_walk_down()

btrfs_find_create_tree_block() may return NULL, so we must check the returned
value, or we will access a NULL pointer.

Signed-off-by: Miao Xie <d7fac3f7b328449401fe64671e732ac315caee28@cn.fujitsu.com>
Signed-off-by: Chris Mason <a169954b4cb1a46cee25f659d3bddfebe02b5fba@oracle.com>

KristFoundation/Programs,KristFoundation/Programs,TeamVee-Kanas/android_kernel_samsung_kanas,TeamVee-Kanas/android_kernel_samsung_kanas,TeamVee-Kanas/android_kernel_samsung_kanas,TeamVee-Kanas/android_kernel_samsung_kanas,KristFoundation/Programs,TeamVee-Kanas/android_kernel_samsung_kanas,KristFoundation/Programs,KristFoundation/Programs,KristFoundation/Programs