ucode-mod-lua: various fixes

Properly handle accesses to properties of the userdatum itself in the
lua_uv_index() __index metamethod and treat integer keys as array indexes
in case of wrapped ucode array values. Also fix an incorrect refcount
decrement in the function.

Also fix uc_lua_vm_get() and uc_lua_lv_getraw() to gracefully handle
accesses to not defined or non-table values and ensure that those functions
properly reset the Lua stack after they complete.

Signed-off-by: Jo-Philipp Wich <jo@mein.io>