fix: add null check after sentry__http_transport_new in default transport

If sentry__http_transport_new returns NULL, the subsequent set_* calls
would dereference a null pointer. Free the client and return NULL early.

Co-Authored-By: Claude Opus 4.6 <noreply@anthropic.com>

Author: jpnurmi

src/transports/sentry_http_transport_curl.c

@@ -247,6 +247,10 @@ sentry__transport_new_default(void)
 
     sentry_transport_t *transport
         = sentry__http_transport_new(client, curl_send_task);
+    if (!transport) {
+        curl_client_free(client);
+        return NULL;
+    }
     sentry__http_transport_set_free_client(transport, curl_client_free);
     sentry__http_transport_set_start_client(transport, curl_client_start);
     return transport;

src/transports/sentry_http_transport_winhttp.c

@@ -324,6 +324,10 @@ sentry__transport_new_default(void)
 
     sentry_transport_t *transport
         = sentry__http_transport_new(client, winhttp_send_task);
+    if (!transport) {
+        winhttp_client_free(client);
+        return NULL;
+    }
     sentry__http_transport_set_free_client(transport, winhttp_client_free);
     sentry__http_transport_set_start_client(transport, winhttp_client_start);
     sentry__http_transport_set_shutdown_client(