inner_join segfaults on NA factor key values

```
library("dplyr")

a <- data.frame(x=c("p", "q", NA), y=c(1, 2, 3), stringsAsFactors=TRUE)
b <- data.frame(x=c("p", "q", "r"), z=c(4,5,6), stringsAsFactors=TRUE)

inner_join(a, b)
```

segfaults for me.
