gve: add missing NULL check for gve_alloc_pending_packet() in TX DQO

In the Linux kernel, the following vulnerability has been resolved:

gve: add missing NULL check for gve_alloc_pending_packet() in TX DQO

gve_alloc_pending_packet() can return NULL, but gve_tx_add_skb_dqo()

did not check for this case before dereferencing the returned pointer.

Add a missing NULL check to prevent a potential NULL pointer

dereference when allocation fails.

This improves robustness in low-memory scenarios.