[BUG]

Error when using `require =` with `verbose = 0` and `multiple = TRUE` in `join()`:

 ```
df1 <- data.frame(x = 1:3, y = 4:6)
df2 <- data.frame(x = 2:5, z = 2:5)
join(df1, df2, require = list(x = 1, y = 1, fail = "error"), verbose = 0, multiple = TRUE)
# Error: object 'mi' not found
```
It seems that, for that combination of parameters, `mi` is not computed in `join()`.
