fix: Properly check error conditions for int-returning functions.

OpenSSL functions returning an int like `EVP_PKEY_derive_init` return
non-positive (0 or negative) integers on an error condition, so we need
to check for both.

See e.g. https://www.openssl.org/docs/man1.1.1/man3/EVP_PKEY_derive.html

src/crypto.cpp

@@ -34,13 +34,21 @@ static const std::size_t SHA256_BLOCK_LENGTH = 64;
 static const std::uint8_t HKDF_DEFAULT_SALT[32] = {};
 
 template <typename T>
-static T checked(T val) {
+inline T checked(T val) {
     if (!val) {
         abort();
     }
     return val;
 }
 
+template <>
+inline int checked(int val) {
+    if (val <= 0) {
+        abort();
+    }
+    return val;
+}
+
 } // namespace
 
 void _olm_crypto_curve25519_generate_key(