Unexpected exception and results with unmatched prefix (or suffix)

The pure versions of regex match extraction functions, `Text.ICU.prefix`, `suffix`, and (possibly) `group` do not correctly handle the case where a group is in a regex but is not used in a match. For example "a(b)?c" against "ac" or "(a)|b" against "b". They assume that `start_` and `end_` return -1 only when the grouping is out of range, but in fact they can when a grouping does not fire.

```
> prefix 1 =<< find (regex [] "abc(def)?ghi") "xabcghiy"
*** Exception: Data.Text.Array.new: size overflow
CallStack (from HasCallStack):
error, called at ./Data/Text/Array.hs:129:20 in text-1.2.2.1-FeA6fTH3E2n883cNXIS2Li:Data.Text.Array
> suffix 1 =<< find (regex [] "abc(def)?ghi") "xabcghiy"
Just "\NULxabcghiy"
```

An out of bounds range gives the expected results:
```
> prefix 2 =<< find (regex [] "abc(def)?ghi") "xabcghiy"
Nothing
> suffix 2 =<< find (regex [] "abc(def)?ghi") "xabcghiy"
Nothing
```

`group` possibly does right thing, but not for the right reason (it extracts -1 to -1), and perhaps should return `Nothing` instead:
```
> group 1 =<< find (regex [] "abc(def)?ghi") "xabcghiy"
Just ""
```

One solution would be to use the safe underlying `start` and `end` functions instead, returning `Nothing` for any underlying `Nothing`. Happy to submit a PR for this approach.