net/i40e: fix address of first segment

This patch fixes (dereference after null check) coverity issue.
The address of first segmented packets was not set correctly during
reassembling packets which led to this issue.

Coverity issue: 343422, 343403
Fixes: ca74903b75cf ("net/i40e: extract non-x86 specific code from vector driver")
Cc: stable@dpdk.org
Signed-off-by: Xiao Zhang <xiao.zhang@intel.com>
Acked-by: Xiaolong Ye <xiaolong.ye@intel.com>