This pr mainly has the following four changes:
1. Add missing lua_pop in `luaGetFromRegistry`.
This bug affects `redis.register_function`, where `luaGetFromRegistry` in
`luaRegisterFunction` will return null when we call `redis.register_function` nested.
.e.g
```
FUNCTION LOAD "#!lua name=mylib \n local lib=redis \n lib.register_function('f2', function(keys, args) lib.register_function('f1', function () end) end)"
fcall f2 0
````
But since we exit when luaGetFromRegistry returns null, it does not cause the stack to grow indefinitely.
3. When getting `REGISTRY_RUN_CTX_NAME` from the registry, use `serverAssert`
instead of error return. Since none of these lua functions are registered at the time
of function load, scriptRunCtx will never be NULL.
4. Add `serverAssert` for `luaLdbLineHook`, `luaEngineLoadHook`.
5. Remove `luaGetFromRegistry` from `redis_math_random` and
`redis_math_randomseed`, it looks like they are redundant.
8aad2ac352