Fix *two* bugs in formatRealFloat:

The first one is in the Haskell 98 errata, namely that negative
exponents cause an infinite loop in the FFFixed case when no precision
is specified.  eg. `Numeric.showFFloat Nothing 0.02 ""'  I've modified
the code in the errata to properly handle the e == 0 case and to be
slightly more efficient when e > 0.

The second bug is this:

   Prelude> Numeric.showFFloat (Just 0) 0.02 ""
   "0."