Broken LOG15_ERROR messages

[tg]

I've been reading the section about error handling and tried to play with some mistyped keys (to see LOG15_ERROR reports), but it didn't work very well.

Here is the code and corresponding output:

log15.Info("one empty error", 1)
// Output:
// t=2015-11-27T19:57:07+0100 lvl=info msg="one empty error" LOG15_ERROR= LOG15_ERROR="Normalized odd number of arguments by adding nil"

log15.Info("empty error", 1, 2)
// Output:  
// t=2015-11-27T19:57:07+0100 lvl=info msg="empty error" LOG15_ERROR=

log15.Info("two empty errors", 1, 2, 3, 4)
// Output:  
// t=2015-11-27T19:57:07+0100 lvl=info msg="two empty errors" LOG15_ERROR= LOG15_ERROR=

[tg]

The problem with empty LOG15_ERROR is due to this code in logfmt:

k, ok := ctx[i].(string)
v := formatLogfmtValue(ctx[i+1])
if !ok {
    k, v = errorKey, formatLogfmtValue(k) // oops, k is empty here!
}

I think the code tried to include key name of invalid field, but this is empty because of the failed conversion to string. One possible solution could be to include field ID in error string, i.e. something like:

v = fmt.Printf("field %d is not a string", i)

I'm happy to provide a patch once the intended behaviour is confirmed.