Fix SIGILL created when ExtendedBuffer is null

When m_ExtendedBuffer is a null pointer, or the
m_StandardIdentity.certificate.type is not "Key", a null buffer is now
returned.

Likely not the correct solution, only fixes SIGILL runtime
interrupt.

src/core/router/identity.cc

@@ -356,6 +356,8 @@ SigningKeyType IdentityEx::GetSigningKeyType() const {
     return bufbe16toh(m_ExtendedBuffer.get());  // signing key
   // TODO(unassigned): should an exception be raised here?
   // since an unsupported setup is used when the above condition is false?
+  std::unique_ptr<std::uint8_t[]> null_key = std::make_unique<std::uint8_t[]>(m_ExtendedLen);
+  return bufbe16toh(null_key.get());  // signing key
 }
 
 CryptoKeyType IdentityEx::GetCryptoKeyType() const {
@@ -364,6 +366,8 @@ CryptoKeyType IdentityEx::GetCryptoKeyType() const {
     return bufbe16toh(m_ExtendedBuffer.get() + 2);  // crypto key
   // TODO(unassigned): should an exception be raised here?
   // since an unsupported setup is used when the above condition is false?
+  std::unique_ptr<std::uint8_t[]> null_key = std::make_unique<std::uint8_t[]>(m_ExtendedLen - 2);
+  return bufbe16toh(null_key.get());  // null crypto key
 }
 
 void IdentityEx::CreateVerifier() const  {