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.

Fixes: a57e5de4 ("gve: DQO: Add TX path")
Signed-off-by: Alok Tiwari <alok.a.tiwari@oracle.com>
Reviewed-by: Mina Almasry <almasrymina@google.com>
Reviewed-by: Simon Horman <horms@kernel.org>
Signed-off-by: David S. Miller <davem@davemloft.net>