NetworkPkg: Fix conditionally uninitialized variables

Fixes CodeQL alerts for CWE-457:
https://cwe.mitre.org/data/definitions/457.html

Cc: Erich McMillan <emcmillan@microsoft.com>
Cc: Jiaxin Wu <jiaxin.wu@intel.com>
Cc: Maciej Rabeda <maciej.rabeda@linux.intel.com>
Cc: Michael D Kinney <michael.d.kinney@intel.com>
Cc: Michael Kubacki <mikuback@linux.microsoft.com>
Cc: Siyuan Fu <siyuan.fu@intel.com>
Co-authored-by: Erich McMillan <emcmillan@microsoft.com>
Signed-off-by: Michael Kubacki <michael.kubacki@microsoft.com>
Reviewed-by: Michael D Kinney <michael.d.kinney@intel.com>
Reviewed-by: Oliver Smith-Denny <osd@smith-denny.com>

NetworkPkg/TcpDxe/TcpInput.c

@@ -1570,6 +1570,9 @@ TcpIcmpInput (
   BOOLEAN     IcmpErrIsHard;
   BOOLEAN     IcmpErrNotify;
 
+  IcmpErrIsHard = FALSE;
+  IcmpErrNotify = FALSE;
+
   if (Nbuf->TotalSize < sizeof (TCP_HEAD)) {
     goto CLEAN_EXIT;
   }