NetworkPkg: Remove ASSERT and use error handling in IpSecDxe

This patch is used to refine the code by removing ASSERT and
using error handling in IpSecDxe driver.

Cc: Ye Ting <ting.ye@intel.com>
Cc: Fu Siyuan <siyuan.fu@intel.com>
Cc: Zhang Lubo <lubo.zhang@intel.com>
Cc: Yao Jiewen <jiewen.yao@intel.com>
Contributed-under: TianoCore Contribution Agreement 1.0
Signed-off-by: Jiaxin Wu <jiaxin.wu@intel.com>
Reviewed-by: Ye Ting <ting.ye@intel.com>

NetworkPkg/IpSecDxe/IpSecConfigImpl.c

@@ -2175,7 +2175,10 @@ IpSecGetVariable (
   VariableNameLength  = StrLen (VariableName);
   VariableNameISize   = (VariableNameLength + 5) * sizeof (CHAR16);
   VariableNameI       = AllocateZeroPool (VariableNameISize);
-  ASSERT (VariableNameI != NULL);
+  if (VariableNameI == NULL) {
+    Status = EFI_OUT_OF_RESOURCES;
+    goto ON_EXIT;
+  }
   
   //
   // Construct the varible name of ipsecconfig meta data.