Bug in date.c

There's a bug in datetime.c when it handles errors converting text into
various date formats. It tries to avoid palloc'ing a cstring copy of the input
by storing it in a stack variable instead but that means it can't handle
inputs over MAXDATELEN. So it throws an error but passes the varlena string
where the format expects a c string. Of course having to generate a c string
for the format begs the question...

The bug can be triggered trivially with:
postgres=# select repeat(' ',64)::text::date;
ERROR: invalid input syntax for type date: " ~!@"

I would be inclined to just go ahead and just call textout which would
effectively be pallocing a copy. Is there some reason these functions in
particular shouldn't leak memory?

I've attached both a patch that does that and a patch that just makes the
minimal fix of calling textout when the error is thrown.

Alternatively it might be handy to have a custom escape in errmsg format
strings for text varlena data.