When gve_alloc_rings fails (such as when failing to acquire MSI-X
vectors or during partial ring allocation), ring cleanup can be
executed multiple times across nested error paths (e.g. within
gve_alloc_rings abort and gve_attach abort).

Because gve_free_counters invoked counter_u64_free without nullifying
the pointer in the stats array, repeated invocation caused a double-free
panic when freeing the same counter references.

Check for non-NULL before freeing and nullify each counter pointer
upon release in gve_free_counters.

Signed-off-by: Jasper Tran O'Leary <jtranoleary@google.com>