Normalisation bug with Hangul / symbols sequence

I re-ran the fuzzing job after #8 was fixed, and it found another case:
```
                   s: [U+c100 U+20d2 U+11c1 U+11c1 "섀⃒ᇁᇁ"]
            toNFC(s): [U+c11a U+20d2 U+11c1 "섚⃒ᇁ"]
            toNFD(s): [U+1109 U+1164 U+20d2 U+11c1 U+11c1 "섀⃒ᇁᇁ"]
     toNFD(toNFC(s)): [U+1109 U+1164 U+11c1 U+20d2 U+11c1 "섚⃒ᇁ"]
```
The last two lines should be equal.