Do not fetch the previous header of the target marker in compact_upto
Do not fetch the previous header of the target marker in compact_upto

- Since we already use 'last_wal_flush_hdr_bid' field, we don't need
to fetch the previous header of the target marker header.

- This may fix a potential bug when there is only one DB header in the
file and compact_upto() is performed on the last header.

Change-Id: I6c5feca14370bc4c664a2fabba93efc5515127e9

couchbase/forestdb,zhaog/forestdb,seem-sky/forestdb,seem-sky/forestdb,wurikiji/forestdb,jinxuan/forestdb,hisundar/forestdb,wurikiji/forestdb,sushmaad/forestdb,zhaog/forestdb,nvoron23/forestdb,sushmaad/forestdb,joliss/forestdb,hisundar/forestdb,nvoron23/forestdb,jinxuan/forestdb,yyangpan/forestdb,yyangpan/forestdb,joliss/forestdb,couchbase/forestdb