Streams: fix backward iteration when entry is not flagged SAMEFIELD.
Streams: fix backward iteration when entry is not flagged SAMEFIELD.

See issue #5006. The comment in the code was also wrong and
was rectified as well.

ctripcorp/redis,neomantra/redis,nnog/redis,ofirluzon/redis,GitHubMota/redis,oranagra/redis,neomantra/redis,oranagra/redis,pmem/redis,charsyam/redis,yossigo/redis,antirez/redis,yossigo/redis,soloestoy/redis,pmem/redis,ofirluzon/redis,antirez/redis,charsyam/redis,charsyam/redis,PKRoma/redis,pmem/redis,soveran/redis,oranagra/redis,charsyam/redis,nnog/redis,charsyam/redis,spinlock/redis,spinlock/redis,soloestoy/redis,nnog/redis,spinlock/redis,neomantra/redis,antirez/redis,yossigo/redis,pmem/redis,soveran/redis,GitHubMota/redis,soveran/redis,ofirluzon/redis,yossigo/redis,soloestoy/redis,soloestoy/redis,neomantra/redis,ctripcorp/redis,antirez/redis,ctripcorp/redis,PKRoma/redis,ctripcorp/redis,nnog/redis,PKRoma/redis,spinlock/redis,PKRoma/redis,neomantra/redis,soloestoy/redis,oranagra/redis,soveran/redis,ofirluzon/redis,GitHubMota/redis,oranagra/redis,ofirluzon/redis,PKRoma/redis,GitHubMota/redis,yossigo/redis