octeon_ep_vf: add NULL check for napi_build_skb()

In the Linux kernel, the following vulnerability has been resolved: octeon_ep_vf: add NULL check for napi_build_skb() napi_build_skb() can return NULL on allocation failure. In __octep_vf_oq_process_rx(), the result is used directly without a NULL check in both the single-buffer and multi-fragment paths, leading to a NULL pointer dereference. Add NULL checks after both napi_build_skb() calls, properly advancing descriptors and consuming remaining fragments on failure.