Joins segfault when by not specified and not found in both

[otsaw]

Impossible joins fail gracefully if by is specified (although something like "Column not found" would be a better error message)

> left_join(data.frame(x=1:5), data.frame(y=1:5), by="x")
Error: index out of bounds
> left_join(data.frame(x=1:5), data.frame(y=1:5), by="y")
Error: index out of bounds

but segfault when it needs to be guessed.

> left_join(data.frame(x=1:5), data.frame(y=1:5))
Joining by: character(0)

 *** caught segfault ***
address (nil), cause 'memory not mapped'

Traceback:
 1: .Call("dplyr_left_join_impl", PACKAGE = "dplyr", x, y, by)
 2: left_join_impl(x, y, by)
 3: left_join.tbl_df(tbl_df(x), y, by = by, copy = copy, ...)
 4: left_join(tbl_df(x), y, by = by, copy = copy, ...)
 5: as.data.frame(left_join(tbl_df(x), y, by = by, copy = copy, ...))
 6: left_join.data.frame(data.frame(x = 1:5), data.frame(y = 1:5))
 7: left_join(data.frame(x = 1:5), data.frame(y = 1:5))

[dayuhuang]

Have the same issue.