net/ixgbe: fix warning with GCC 9

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: 8eecb3295aed ("ixgbe: add LRO support")
Cc: stable@dpdk.org
Signed-off-by: Bruce Richardson <bruce.richardson@intel.com>