bug in printf, and patch

Text.Printf.printf has a bug where it doesn't like negative numbers
for * modifiers.  I opened a ticket along with a patch at

http://hackage.haskell.org/trac/ghc/ticket/7457

I feel like it shouldn't need a discussion since it's just a bug fix,
but if someone wants to speak up go ahead.

It will change behaviour for people using negative numbers, but since
the previous behaviour was "doesn't work" I hope they won't mind :)