Segfault in left_join when factor levels not equal

Similar to #371 (`R --vanilla`)

Minimal: 

``` r
library(dplyr)
d1 <- iris[, c("Species", "Sepal.Length")]
d2 <- iris[, c("Species", "Sepal.Width")]
d2$Species <- factor(as.character(d2$Species), levels=rev(levels(d1$Species)))
left_join(d1, d2, by="Species")
```
