rework synchronized block in TypeDeserializerBase (#4472)

Author: pjfanning

src/main/java/com/fasterxml/jackson/databind/jsontype/impl/TypeDeserializerBase.java

@@ -223,13 +223,15 @@ protected final JsonDeserializer<Object> _findDefaultImplDeserializer(Deserializ
             return NullifyingDeserializer.instance;
         }
 
-        synchronized (_defaultImpl) {
-            if (_defaultImplDeserializer == null) {
-                _defaultImplDeserializer = ctxt.findContextualValueDeserializer(
+        if (_defaultImplDeserializer == null) {
+            synchronized (_defaultImpl) {
+                if (_defaultImplDeserializer == null) {
+                    _defaultImplDeserializer = ctxt.findContextualValueDeserializer(
                         _defaultImpl, _property);
+                }
             }
-            return _defaultImplDeserializer;
         }
+        return _defaultImplDeserializer;
     }
 
     /**