Text builder instance writes to wrong offset

Consider the following program:

``` Haskell
import Data.Double.Conversion.Convertable
import Data.Text
import qualified Data.Text.Lazy.Builder as TLB

main :: IO ()
main = do
let
x1 :: Text = "I have " <> toExponential 2 (3.14 :: Double) <> " cakes."
x2 :: TLB.Builder = "I have " <> toExponential 2 (3.14 :: Double) <> " cakes."
putStrLn $ show x1
putStrLn $ show x2
```

The output is ...
```
"I have 3.14e0 cakes."
"3.14e0 \NUL\NUL\NUL\NUL\NUL\NUL cakes."
```
The `Text` instance of `Convertable` produces correct output, but the `Builder` instance does not. In some other tests I did the Builder output always gets written to the beginning instead of the correct location.