net/bnxt: fix crash in vector mode Tx

net/bnxt: fix crash in vector mode Tx

rte_pktmbuf_prefree_seg() can return NULL if the mbuf still
has remaining references on it.

Adding a NULL check to prevent segfault.

Fixes: bc4a000f2f53 ("net/bnxt: implement SSE vector mode")
Fixes: 398358341419 ("net/bnxt: support NEON")
Cc: stable@dpdk.org
Signed-off-by: Kalesh AP <kalesh-anakkur.purayil@broadcom.com>
Reviewed-by: Lance Richardson <lance.richardson@broadcom.com>
Reviewed-by: Somnath Kotur <somnath.kotur@broadcom.com>
Reviewed-by: Ajit Khaparde <ajit.khaparde@broadcom.com>