Avoid abort in combinePubKeys (#31)

Fixes #27.

src/Crypto/Secp256k1.hs

@@ -482,15 +482,18 @@ tweakMulPubKey (PubKey fp) (Tweak ft) = withContext $ \ctx ->
 
 -- | Add multiple public keys together.
 combinePubKeys :: [PubKey] -> Maybe PubKey
-combinePubKeys pubs = withContext $ \ctx -> pointers [] pubs $ \ps ->
-    allocaArray (length ps) $ \a -> do
-        pokeArray a ps
-        fp <- mallocForeignPtr
-        ret <- withForeignPtr fp $ \p ->
-            ecPubKeyCombine ctx p a (fromIntegral $ length ps)
-        if isSuccess ret
-            then return $ Just $ PubKey fp
-            else return Nothing
+combinePubKeys pubs = withContext $ \ctx ->
+    if pubs == []
+        then return Nothing
+        else pointers [] pubs $ \ps ->
+            allocaArray (length ps) $ \a -> do
+                pokeArray a ps
+                fp <- mallocForeignPtr
+                ret <- withForeignPtr fp $ \p ->
+                    ecPubKeyCombine ctx p a (fromIntegral $ length ps)
+                if isSuccess ret
+                    then return $ Just $ PubKey fp
+                    else return Nothing
   where
     pointers ps [] f = f ps
     pointers ps (PubKey fp : pubs') f =

test/Crypto/Secp256k1Spec.hs

@@ -57,6 +57,7 @@ spec = do
         it "add public key" $ property $ tweakAddPubKeyTest
         it "multiply public key" $ property $ tweakMulPubKeyTest
         it "combine public keys" $ property $ combinePubKeyTest
+        it "can't combine 0 public keys" $ property $ combinePubKeyEmptyListTest
         it "negates tweak" $ property $ negateTweakTest
 #ifdef ECDH
     describe "ecdh" $ do
@@ -277,6 +278,13 @@ combinePubKeyTest =
     expected = importPubKey $ hexToBytes
         "043d9a7ec70011efc23c33a7e62d2ea73cca87797e3b659d93bea6aa871aebde56c3bc6134ca82e324b0ab9c0e601a6d2933afe7fb5d9f3aae900f5c5dc6e362c8"
 
+combinePubKeyEmptyListTest :: Assertion
+combinePubKeyEmptyListTest =
+    assertEqual "empty pubkey list must return Nothing" expected combined
+  where
+    expected = Nothing
+    combined = combinePubKeys []
+
 negateTweakTest :: Assertion
 negateTweakTest =
     assertEqual "can recover secret key 1 after adding tweak 1" oneKey subtracted