ethdev: fix crash if malloc of user callback fails
ethdev: fix crash if malloc of user callback fails

Found by coccinelle script.
If rte_zmalloc() failed in rte_eth_dev_callback_register
then NULL pointer would be dereferenced.

Signed-off-by: Stephen Hemminger <06fa905d7f2aaced6dc72e9511c71a2a51e8aead@networkplumber.org>
Acked-by: Thomas Monjalon <ee2fa8b2681c737f775687e475dcbf2028a412f1@6wind.com>
[Thomas: restore pointer comparison style]

tsphillips/dpdk-fork,msune/dpdk,john-mcnamara-intel/dpdk,msune/dpdk,john-mcnamara-intel/dpdk,mixja/dpdk,venkynv/dpdk-mirror,mixja/dpdk,venkynv/dpdk-mirror,msune/dpdk,venkynv/dpdk-mirror,john-mcnamara-intel/dpdk,msune/dpdk,john-mcnamara-intel/dpdk,tsphillips/dpdk-fork,mixja/dpdk,tsphillips/dpdk-fork,mixja/dpdk,venkynv/dpdk-mirror,tsphillips/dpdk-fork