net/ixgbe: fix address of first segment

net/ixgbe: 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: 13245
Fixes: 8a44c15aa57d ("net/ixgbe: extract non-x86 specific code from vector driver")
Cc: stable@dpdk.org
Signed-off-by: Joyce Kong <joyce.kong@arm.com>
Reviewed-by: Xiao Zhang <xiao.zhang@intel.com>
Reviewed-by: Xiaolong Ye <xiaolong.ye@intel.com>