Remove assert(current_ == CurrentReverse()) in MergingIterator::Prev()
Remove assert(current_ == CurrentReverse()) in MergingIterator::Prev()

Summary:
Remove assert(current_ == CurrentReverse()) in MergingIterator::Prev()
because it is possible to have some keys larger than the seek-key
inserted between Seek() and SeekToLast(), which makes current_ not
equal to CurrentReverse().

Test Plan: db_stress

Reviewers: igor, sdong, IslamAbdelRahman, anthony

Reviewed By: anthony

Subscribers: dhruba, leveldb

Differential Revision: https://reviews.facebook.net/D41331

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-Projects/rocksdb.replicated,JohnPJenkins/rocksdb,SunguckLee/RocksDB,Vaisman/rocksdb,Applied-Duality/rocksdb,bbiao/rocksdb,biddyweb/rocksdb,luckywhu/rocksdb,fengshao0907/rocksdb,Vaisman/rocksdb,Applied-Duality/rocksdb,anagav/rocksdb,ylong/rocksdb,tsheasha/rocksdb,dkorolev/rocksdb,wenduo/rocksdb,facebook/rocksdb,tsheasha/rocksdb,alihalabyah/rocksdb,jalexanderqed/rocksdb,mbarbon/rocksdb,zhangpng/rocksdb,skunkwerks/rocksdb,wat-ze-hex/rocksdb,lgscofield/rocksdb,wlqGit/rocksdb,bbiao/rocksdb,caijieming-baidu/rocksdb,luckywhu/rocksdb,tizzybec/rocksdb,vashstorm/rocksdb,mbarbon/rocksdb,vmx/rocksdb,lgscofield/rocksdb,ylong/rocksdb