Fix use-after-free in nsigner_client_call error handling that caused segfaults on RPC error responses

VERSION

@@ -1 +1 @@
-0.6.14
+0.6.15

nostr_core/nostr_core.h

@@ -2,10 +2,10 @@
 #define NOSTR_CORE_H
 
 // Version information (auto-updated by increment_and_push.sh)
-#define VERSION "v0.6.14"
+#define VERSION "v0.6.15"
 #define VERSION_MAJOR 0
 #define VERSION_MINOR 6
-#define VERSION_PATCH 14
+#define VERSION_PATCH 15
 
 /*
  * NOSTR Core Library - Complete API Reference

nostr_core/nsigner_client.c

@@ -339,7 +339,9 @@ int nsigner_client_call(nsigner_client_t* client,
     error_item = cJSON_GetObjectItemCaseSensitive(res, "error");
     if (cJSON_IsObject(error_item)) {
         int rpc_code = 0;
+        char rpc_msg_buf[128];
         const char* rpc_msg = "rpc_error";
+        int mapped_rc;
 
         code_item = cJSON_GetObjectItemCaseSensitive(error_item, "code");
         message_item = cJSON_GetObjectItemCaseSensitive(error_item, "message");
@@ -350,9 +352,18 @@ int nsigner_client_call(nsigner_client_t* client,
             rpc_msg = message_item->valuestring;
         }
 
-        nsigner_client_set_error(client, rpc_msg);
+        /* Copy the error message to a local buffer BEFORE freeing res,
+         * because rpc_msg points into the cJSON tree and would be a
+         * dangling pointer after cJSON_Delete(res). This was a
+         * use-after-free that caused segfaults on error responses
+         * (e.g. path_not_allowed). */
+        strncpy(rpc_msg_buf, rpc_msg, sizeof(rpc_msg_buf) - 1);
+        rpc_msg_buf[sizeof(rpc_msg_buf) - 1] = '\0';
+
+        nsigner_client_set_error(client, rpc_msg_buf);
+        mapped_rc = nsigner_client_map_rpc_error(rpc_code, rpc_msg_buf);
         cJSON_Delete(res);
-        return nsigner_client_map_rpc_error(rpc_code, rpc_msg);
+        return mapped_rc;
     }
 
     result_item = cJSON_GetObjectItemCaseSensitive(res, "result");