btrfs: fix uninitialized variable in find_first_clear_extent_bit

This was caught when syncing extent-io-tree.c into btrfs-progs.  This
however isn't really a problem, the only way next would be uninitialized
is if we found the range we were looking for, and in this case we don't
care about next.  However it's a compile error, so fix it up.

Signed-off-by: Josef Bacik <josef@toxicpanda.com>
Signed-off-by: David Sterba <dsterba@suse.com>