net/ixgbe: fix warning with GCC 9

[ upstream commit e81b056 ]

Compiling on Fedora 30, we get the following warning, causing build failure
when Werror flag is set:

../drivers/net/ixgbe/ixgbe_rxtx.c:2141:14: warning:
‘nmb’ may be used uninitialized in this function [-Wmaybe-uninitialized]
 2141 |    rxe->mbuf = nmb;
      |    ~~~~~~~~~~^~~~~

Initializing the value to "NULL" fixes the issue.

Fixes: 8eecb32 ("ixgbe: add LRO support")

Signed-off-by: Bruce Richardson <bruce.richardson@intel.com>

drivers/net/ixgbe/ixgbe_rxtx.c

@@ -2029,7 +2029,7 @@ ixgbe_recv_pkts_lro(void *rx_queue, struct rte_mbuf **rx_pkts, uint16_t nb_pkts,
 		struct ixgbe_rx_entry *next_rxe = NULL;
 		struct rte_mbuf *first_seg;
 		struct rte_mbuf *rxm;
-		struct rte_mbuf *nmb;
+		struct rte_mbuf *nmb = NULL;
 		union ixgbe_adv_rx_desc rxd;
 		uint16_t data_len;
 		uint16_t next_id;