xfsprogs: skip freelist scans of corrupt agf

xfsprogs: skip freelist scans of corrupt agf

If an agf has bad values in the freelist, this can wreak
havoc if, for example, first > last and the loop
never exits; we index agfl->agfl_bno[i] off into the weeds.

If they're off, warn about it and skip the scan.

This is done both in xfs_check and xfs_db's freespace cmd.

Also fix uninit'd variable "i" from previous, similar fix
for xfs_repair.

Signed-off-by: Eric Sandeen <sandeen@redhat.com>
Reviewed-by: Dave Chinner <dchinner@redhat.com>
Signed-off-by: Rich Johnston <rjohnston@sgi.com>