NetworkPkg/DnsDxe: Avoid to access the freed memory buffer.

NetworkPkg/DnsDxe: Avoid to access the freed memory buffer.

The HostNameToIp() is a asynchronous function, so the caller
may free the HostName buffer immediately once HostNameToIp()
is returned. Then DNS driver may access the freed memory buffer
later.

This patch is to fix above issue.

Cc: Ye Ting <ting.ye@intel.com>
Cc: Fu Siyuan <siyuan.fu@intel.com>
Cc: Wang Fan <fan.wang@intel.com>
Contributed-under: TianoCore Contribution Agreement 1.0
Signed-off-by: Wu Jiaxin <jiaxin.wu@intel.com>
Reviewed-by: Fu Siyuan <siyuan.fu@intel.com>