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.