Fix crash when passing invalid key to Crypto.

modules/mbedtls/crypto_mbedtls.cpp

@@ -237,6 +237,7 @@ Ref<CryptoKey> CryptoMbedTLS::generate_rsa(int p_bytes) {
 
 Ref<X509Certificate> CryptoMbedTLS::generate_self_signed_certificate(Ref<CryptoKey> p_key, String p_issuer_name, String p_not_before, String p_not_after) {
 	Ref<CryptoKeyMbedTLS> key = static_cast<Ref<CryptoKeyMbedTLS> >(p_key);
+	ERR_FAIL_COND_V_MSG(key.is_null(), NULL, "Invalid private key argument.");
 	mbedtls_x509write_cert crt;
 	mbedtls_x509write_crt_init(&crt);