btrfs-progs: Fix an infinite loop in btrfs_next_bg

btrfs-progs: Fix an infinite loop in btrfs_next_bg

I've run into a couple filesystems where btrfs-find-root would spin
indefinitely.

If the first cache extent start location is 0, we end up in an infinite
loop in btrfs_next_bg().  Fix it by checking for that situation, and
jumping to the next bg if necessary.

Fixes: e2e0dae9 (btrfs-progs: volume: Fix a bug causing btrfs-find-root to skip first chunk)
Signed-off-by: Justin Maggard <jmaggard@netgear.com>
Signed-off-by: David Sterba <dsterba@suse.com>