MB-6771 Fix handling of couchstore_last_os_error()
MB-6771 Fix handling of couchstore_last_os_error()

couchstore_last_os_error will fill the destination buffer with a
null-terminated string, and may not fill it completely. Should use the
std::string constructor that accepts null-terminated strings, rather
than consuming possible garbage in the buffer.

Change-Id: I3d061f1e656435e84271a7b28631b95861c15ce7
Reviewed-on: http://review.couchbase.org/21437
Reviewed-by: Jin Lim <c45cd80a556f432a3607171a6818cd5a074b5cd4@couchbase.com>
Tested-by: buildbot <80754af91bfb6d1073585b046fe0a474ce868509@couchbase.com>

jimwwalker/ep-engine,couchbase/ep-engine,couchbase/ep-engine,hisundar/ep-engine,daverigby/kv_engine,abhinavdangeti/ep-engine,sriganes/ep-engine,hisundar/ep-engine,couchbase/ep-engine,abhinavdangeti/ep-engine,membase/ep-engine,jimwwalker/ep-engine,couchbase/ep-engine,teligent-ru/ep-engine,abhinavdangeti/ep-engine,daverigby/ep-engine,daverigby/kv_engine,sriganes/ep-engine,hisundar/ep-engine,abhinavdangeti/ep-engine,teligent-ru/ep-engine,owendCB/ep-engine,membase/ep-engine,jimwwalker/ep-engine,daverigby/kv_engine,teligent-ru/ep-engine,daverigby/ep-engine,daverigby/ep-engine,owendCB/ep-engine,owendCB/ep-engine,daverigby/kv_engine,sriganes/ep-engine,membase/ep-engine,hisundar/ep-engine,jimwwalker/ep-engine,teligent-ru/ep-engine,abhinavdangeti/ep-engine,membase/ep-engine,sriganes/ep-engine,daverigby/ep-engine,owendCB/ep-engine