osd: don't open deleted map from generate_past_intervals

osd: don't open deleted map from generate_past_intervals

The first get_map() call needs to be avoided when stop < last_epoch.  This
fixes a crash like

2011-11-08 21:51:09.046739 7fcf6e035700 osd.0 5 pg[1.1p0( empty n=0 ec=1 les/c 3/3 2/2/2) [0,1] r=0 mlcod 0'0 !hml peering] enter Started/Primary/Peering/GetInfo
2011-11-08 21:51:09.046767 7fcf6e035700 osd.0 5 pg[1.1p0( empty n=0 ec=1 les/c 3/3 2/2/2) [0,1] r=0 mlcod 0'0 !hml peering] generate_past_intervals over epochs 4-1
2011-11-08 21:51:09.046796 7fcf6e035700 osd.0 5 get_map 1 - loading and decoding 0x183b000
*** Caught signal (Aborted) **
 in thread 7fcf6e035700
 ceph version 0.37-327-g1bc1a24 (commit:1bc1a244dbf7342662322d002d0c9d41ad5fee6f)

...

Signed-off-by: Sage Weil <sage@newdream.net>