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().