List in last line of do-block prevents formatting

Nobody has claimed this yet.

I have the following function:

```haskell
wnums :: [Double] -> [Double]
wnums nums = do
a <- nums
b <- nums \\ [a]
c <- nums \\ [a, b]
d <- nums \\ [a, b, c]
o1 <- opts
o2 <- opts
o3 <- opts
[ (a `o1` b) `o2` (c `o3` d)
, ((a `o1` b) `o2` c) `o3` d
, a `o1` (b `o2` (c `o3` d))
, (a `o1` (b `o2` c)) `o3` d
, a `o1` ((b `o2` c) `o3` d)
]
```

This function compiles but when i run brittany on the source file then it errors out with `ERROR: brittany pretty printer returned syntactically invalid result.`

Without this function the source file can be reformatted just fine.

Reproduce the failure by adding the Haskell function from the issue to a source file and running brittany on it. Inspect the formatter output and its syntax validity; done means this do-block formats successfully without the syntactically invalid result error.

Written by the indexing model from the issue text.