NetworkPkg: Code logic optimization for DnsDxe and HttpDxe driver

NetworkPkg: Code logic optimization for DnsDxe and HttpDxe driver

Revise some errors which may lead NULL pointer be dereferenced:
* DnsDhcp.c: Paralist may be used without any initialized
* DnsHeader and RcvString may be null but be dereferenced
* HttpDriver.c: revise an if judgment
* HttpImpl.c: add a judgment to avoid null dereferenced

Contributed-under: TianoCore Contribution Agreement 1.0
Signed-off-by: fanwang2 <fan.wang@intel.com>
Reviewed-by: Ye Ting <ting.ye@intel.com>
Reviewed-by: Jiaxin Wu <jiaxin.wu@intel.com>
git-svn-id: https://svn.code.sf.net/p/edk2/code/trunk/edk2@17923 6f19259b-4bc3-4df7-8a09-765794883524