Initialized a bool variable to fix warning (#26947)

Author: jiayi11

src/System.Private.Xml/src/System/Xml/Serialization/XmlSerializationReader.cs

@@ -111,7 +111,7 @@ public abstract class XmlSerializationReader : XmlSerializationGeneratedCode
         private string _guidID;
         private string _timeSpanID;
 
-        private static bool s_checkDeserializeAdvances;
+        private static bool s_checkDeserializeAdvances=false;
 
         protected abstract void InitIDs();