btrfs: add a sanity check for btrfs root in btrfs_search_slot()

In the Linux kernel, the following vulnerability has been resolved:

btrfs: add a sanity check for btrfs root in btrfs_search_slot()

Syzbot reports a null-ptr-deref in btrfs_search_slot().

The reproducer is using rescue=ibadroots, and the extent tree root is

corrupted thus the extent tree is NULL.

When scrub tries to search the extent tree to gather the needed extent

info, btrfs_search_slot() doesn't check if the target root is NULL or

not, resulting the null-ptr-deref.

Add sanity check for btrfs root before using it in btrfs_search_slot().