Fix LUA_OBJCACHE segfault.

When scanning the argument list inside of a redis.call() invocation
for pre-cached values, there was no check being done that the
argument we were on was in fact within the bounds of the cache size.

So if a redis.call() command was ever executed with more than 32
arguments (current cache size #define setting) redis-server could
segfault.
Merge pull request #1764 from michael-grunder/lua_cache_segfault

Fix LUA_OBJCACHE segfault.