osd/PG: fix assert when deep repair finds no errors

If there are no deep repairs, we don't want to assert.
Fixes:

    -1> 2014-02-21 21:13:56.393087 7f0258ff9700  0 log [INF] : 0.0 repair ok, 0 fixed
     0> 2014-02-21 21:13:56.428703 7f0258ff9700 -1 osd/PG.cc: In function 'void PG::scrub_finish()' thread 7f0258ff9700 time 2014-02-21 21:13:56.393127
osd/PG.cc: 4294: FAILED assert(deep_scrub)

Signed-off-by: David Zafman <david.zafman@inktank.com>
Signed-off-by: Sage Weil <sage@inktank.com>