move some unsafe code inside an unsafe{} boundary

An internal function had a non-unsafe signature but could be called
with data that would cause it to exhibit UB. Move the unsafety inside
of the function so that the function signature now enforces soundness.

Fixes #481

src/ecdsa/mod.rs

@@ -257,10 +257,14 @@ impl<C: Signing> Secp256k1<C> {
         &self,
         msg: &Message,
         sk: &SecretKey,
-        noncedata_ptr: *const ffi::types::c_void,
+        noncedata: Option<&[u8; 32]>,
     ) -> Signature {
         unsafe {
             let mut ret = ffi::Signature::new();
+            let noncedata_ptr = match noncedata {
+                Some(arr) => arr.as_c_ptr() as *const _,
+                None => ptr::null(),
+            };
             // We can assume the return value because it's not possible to construct
             // an invalid signature from a valid `Message` and `SecretKey`
             assert_eq!(ffi::secp256k1_ecdsa_sign(self.ctx, &mut ret, msg.as_c_ptr(),
@@ -273,7 +277,7 @@ impl<C: Signing> Secp256k1<C> {
     /// Constructs a signature for `msg` using the secret key `sk` and RFC6979 nonce
     /// Requires a signing-capable context.
     pub fn sign_ecdsa(&self, msg: &Message, sk: &SecretKey) -> Signature {
-        self.sign_ecdsa_with_noncedata_pointer(msg, sk, ptr::null())
+        self.sign_ecdsa_with_noncedata_pointer(msg, sk, None)
     }
 
     /// Constructs a signature for `msg` using the secret key `sk` and RFC6979 nonce
@@ -287,8 +291,7 @@ impl<C: Signing> Secp256k1<C> {
         sk: &SecretKey,
         noncedata: &[u8; 32],
     ) -> Signature {
-        let noncedata_ptr = noncedata.as_ptr() as *const ffi::types::c_void;
-        self.sign_ecdsa_with_noncedata_pointer(msg, sk, noncedata_ptr)
+        self.sign_ecdsa_with_noncedata_pointer(msg, sk, Some(noncedata))
     }
 
     fn sign_grind_with_check(