(task_content.c) Fix possible double free()

content_zip_file_decompressed_handle() didn't set the handler's data
member to NULL after freeing on error. That causes problem when the function fails and
content_zip_file_decompressed() tries to free it afterwards.

tasks/task_content.c

@@ -525,6 +525,9 @@ error:
    if (handle->data)
       free(handle->data);
 
+   handle->stream = NULL;
+   handle->data   = NULL;
+
    return false;
 }