CLUSTER GETKEYSINSLOT: avoid overallocating.
CLUSTER GETKEYSINSLOT: avoid overallocating.

Close #3911.

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