tracing: Use a copy of the va_list for _assignvstr()
This is an automated ID intended to aid in discovery of potential security vulnerabilities. The actual impact and attack plausibility have not yet been proven. This ID is fixed in Linux Kernel version v5.19.2 by commit 4ee7e7dd1f9931cbba720c0df83fcdf03bc0f75a, it was introduced in version v5.19.2 by commit 351db9a0357428384ba15fc1968f39d5e7693f6b. For more details please see the references link.