lua_struct.c/getnum: throw error if overflow happen
lua_struct.c/getnum: throw error if overflow happen

Fix issue #2855

maodeyi/redis_lua,splitice/redis,ramonsnir/redis,slfs007/mk-redis,louisliangjun/redis,splitice/redis,ramonsnir/redis,slfs007/mk-redis,splitice/redis,ramonsnir/redis,louisliangjun/redis,splitice/redis,maodeyi/redis_lua,slfs007/mk-redis,louisliangjun/redis,louisliangjun/redis,maodeyi/redis_lua