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/HttpDxe/HttpImpl.c

@@ -766,6 +766,10 @@ HttpResponseWorker (
   HTTP_TOKEN_WRAP               *ValueInItem;
   UINTN                         HdrLen;
 
+  if (Wrap == NULL || Wrap->HttpInstance == NULL) {
+    return EFI_INVALID_PARAMETER;
+  }
+  
   HttpInstance = Wrap->HttpInstance;
   Token = Wrap->HttpToken;