net: ethtool: not call vzalloc for zero sized memory request

net: ethtool: not call vzalloc for zero sized memory request

NULL or ZERO_SIZE_PTR will be returned for zero sized memory
request, and derefencing them will lead to a segfault

so it is unnecessory to call vzalloc for zero sized memory
request and not call functions which maybe derefence the
NULL allocated memory

this also fixes a possible memory leak if phy_ethtool_get_stats
returns error, memory should be freed before exit

Signed-off-by: Li RongQing <[email protected]>
Reviewed-by: Wang Li <[email protected]>
Reviewed-by: Michal Kubecek <[email protected]>
Signed-off-by: David S. Miller <[email protected]>