gve: fix for null pointer dereference.

Avoid passing NULL skb to __skb_put() function call if
napi_alloc_skb() returns NULL.

Fixes: 37149e93

 ("gve: Implement packet continuation for RX.")
Signed-off-by: Ameer Hamza <amhamza.mgc@gmail.com>
Link: https://lore.kernel.org/r/20211205183810.8299-1-amhamza.mgc@gmail.com
Signed-off-by: Jakub Kicinski <kuba@kernel.org>