net/9p/usbg: fix handling of the failed kzalloc() memory allocation

In the Linux kernel, the following vulnerability has been resolved: net/9p/usbg: fix handling of the failed kzalloc() memory allocation On the linux-next, next-20241108 vanilla kernel, the coccinelle tool gave the following error report: ./net/9p/trans_usbg.c:912:5-11: ERROR: allocation function on line 911 returns NULL not ERR_PTR on failure kzalloc() failure is fixed to handle the NULL return case on the memory exhaustion.