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/DnsImpl.c

@@ -1049,6 +1049,7 @@ IsValidDnsResponse (
       continue;
     } else {
       TxString = NetbufGetByte (Packet, 0, NULL);
+      ASSERT (TxString != NULL);
       DnsHeader = (DNS_HEADER *) TxString;
       QueryName = (CHAR8 *) (TxString + sizeof (*DnsHeader));
       QuerySection = (DNS_QUERY_SECTION *) (QueryName + AsciiStrLen (QueryName) + 1);
@@ -1426,7 +1427,8 @@ DnsOnPacketReceived (
   ASSERT (Packet != NULL);
   
   RcvString = NetbufGetByte (Packet, 0, NULL);
-
+  ASSERT (RcvString != NULL);
+  
   //
   // Parse Dns Response
   //