Fix key extraction for z{union,inter}store
Fix key extraction for z{union,inter}store

The previous implementation wasn't taking into account
the storage key in position 1 being a requirement (it
was only counting the source keys in positions 3 to N).

Fixes antirez/redis#1581

ksarch-saas/redis,ttuna/msot-redis,Hailei/redis,MSOpenTech/redis,maodeyi/redis_lua,jxwr/redis,MSOpenTech/redis,HunanTV/redis,Hailei/redis,louisliangjun/redis,xuzhezhaozhao/redis_reading,splitice/redis,ksarch-saas/redis,jxwr/redis,darksideofthemoo/redis-histogram,slfs007/mk-redis,darksideofthemoo/redis-histogram,spearhead-ea/redis,maodeyi/redis_lua,ramonsnir/redis,ttuna/msot-redis,MSOpenTech/redis,spearhead-ea/redis,himoca/redis,splitice/redis,ramonsnir/redis,xuzhezhaozhao/redis_reading,HunanTV/redis,ttuna/msot-redis,Hailei/redis,louisliangjun/redis,HunanTV/redis,spearhead-ea/redis,jxwr/redis,ramonsnir/redis,ksarch-saas/redis,xuzhezhaozhao/redis_reading,maodeyi/redis_lua,ksarch-saas/redis,himoca/redis,MSOpenTech/redis,louisliangjun/redis,slfs007/mk-redis,MSOpenTech/redis,splitice/redis,HunanTV/redis,slfs007/mk-redis,ttuna/msot-redis,splitice/redis,himoca/redis,darksideofthemoo/redis-histogram,Hailei/redis,ttuna/msot-redis,xuzhezhaozhao/redis_reading,darksideofthemoo/redis-histogram,louisliangjun/redis