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

NetworkPkg/DnsDxe/DnsDhcp.c

@@ -301,6 +301,8 @@ GetDns4ServerFromDhcp4 (
   Data                       = NULL;
   InterfaceInfo              = NULL;
 
+  ZeroMem ((UINT8 *) ParaList, sizeof (ParaList));
+
   ZeroMem (&MnpConfigData, sizeof (EFI_MANAGED_NETWORK_CONFIG_DATA));
   
   ZeroMem (&DnsServerInfor, sizeof (DNS4_SERVER_INFOR));