cid#1500546 silence Explicit null dereferenced

Change-Id: I44b97965548bd84adf6dc3ec1c67f028a4b31075
Reviewed-on: https://gerrit.libreoffice.org/c/core/+/131000
Tested-by: Jenkins
Reviewed-by: Caolán McNamara <caolanm@redhat.com>

svx/source/customshapes/EnhancedCustomShapeFunctionParser.cxx

@@ -901,6 +901,8 @@ public:
         std::shared_ptr<ExpressionNode> pFirstArg( std::move(rNodeStack.top()) );
         rNodeStack.pop();
 
+        assert(pThirdArg && pSecondArg && pFirstArg);
+
         // create combined ExpressionNode
         auto pNode = std::make_shared<IfExpression>( pFirstArg, pSecondArg, pThirdArg );
         // check for constness