Network Service crash fix in network::mojom::SSLPrivateKeyProxy::Sign

This CL protects calling Sign() on a private key that is null. The key
can become null in two cases.  The first case is if |network_service_client_|
is null during OnCertificateRequested(). The second case is if
HandleSSLPrivateKeyError() is called.

In both cases, after this CL, if |ssl_private_key_| is null, we just call
the sign callback with net::ERR_SSL_CLIENT_AUTH_CERT_NO_PRIVATE_KEY.

Bug: 887539
Cq-Include-Trybots: luci.chromium.try:linux_mojo
Change-Id: I0e2563f00d73ff0659786e85a041567ceeaaea48
Reviewed-on: https://chromium-review.googlesource.com/1237396Reviewed-by: John Abd-El-Malek <jam@chromium.org>
Commit-Queue: Doug Turner <dougt@chromium.org>
Cr-Commit-Position: refs/heads/master@{#593036}

services/network/url_loader.cc

@@ -245,7 +245,7 @@ class SSLPrivateKeyInternal : public net::SSLPrivateKey {
             base::span<const uint8_t> input,
             net::SSLPrivateKey::SignCallback callback) override {
     std::vector<uint8_t> input_vector(input.begin(), input.end());
-    if (ssl_private_key_.encountered_error()) {
+    if (!ssl_private_key_ || ssl_private_key_.encountered_error()) {
       base::ThreadTaskRunnerHandle::Get()->PostTask(
           FROM_HERE,
           base::BindOnce(std::move(callback),