xfs_repair: don't let bplist index go negative in prefetch

xfs_repair: don't let bplist index go negative in prefetch

After:

bbd3275 repair: don't unlock prefetch tree to read discontig buffers

Coverity spotted that it's possible for us to arrive at the loop
below with num == 1, and then we decrement it to 0, and try to
index bplist[num-1].

I think this was possible before the change, i.e. it's probably
not a regression.

Fix this by not trying to shrink the window unless we have
more than one buffer in the array.

Signed-off-by: Eric Sandeen <sandeen@redhat.com>
Reviewed-by: Dave Chinner <dchinner@redhat.com>
Signed-off-by: Dave Chinner <david@fromorbit.com>