Fix CID 1058791

rgw_rados.cc: fix invalid iterator comparison 
The iterator should be compared against the end() function of
the same iter() from region_conn_map.

CID 1058791 (#1 of 1): Invalid iterator comparison (MISMATCHED_ITERATOR)
  mismatched_comparison: Comparing "iter" from "this->region_conn_map" to
  "this->zone_conn_map.end()" from "this->zone_conn_map".