From srfi/19, string->date internally fails on certain inputs

[Alexis King]

Reported by Jordan Johnson via the users mailing list here. It seems that string->date fails in some way when parsing certain format strings.

(require srfi/19)
(string->date "Sun, 02 Feb 2015" "~a, ~d ~b ~Y")

result arity mismatch;
expected number of values not received
 expected: 1
 received: 0
 values...:

[Robby Findler]

Thanks, I've pushed the linked commit.