Refactor schnorrSign: Remove unsafePerformIO, improve safety

Author: prolic

src/Crypto/Secp256k1.hs

@@ -755,9 +755,9 @@ schnorrSign mGen KeyPair{..} bs
         keyPairPtr <- ContT (withForeignPtr keyPairFPtr)
         lift $ do
             sigBuf <- mallocBytes 64
-            let randomPtr = case mGen of
-                    Just gen -> castPtr . unsafePerformIO . BS.useAsCString (BS.pack $ Prelude.take 32 $ randoms gen) $ return
-                    Nothing -> nullPtr
+            randomPtr <- case mGen of
+                Just gen -> fmap castPtr $ BS.useAsCString (BS.pack $ Prelude.take 32 $ randoms gen) return
+                Nothing -> pure nullPtr
             ret <- Prim.schnorrsigSign ctx sigBuf msgHashPtr keyPairPtr randomPtr
             if isSuccess ret
                 then Just . SchnorrSignature <$> newForeignPtr finalizerFree sigBuf