QML Inspector: Don't misinterpret objects with ID 0

The invalid ID is -1 and we shouldn't send 0 to the engine debug
service by default as that will select some random object as context
for expression evaluation.

Change-Id: Ide245468f787cc449ff50ac846c7ff31620c01b4
Task-number: QTCREATORBUG-14931
Reviewed-by: hjk <hjk@theqtcompany.com>

src/plugins/debugger/watchdata.cpp

@@ -119,7 +119,7 @@ bool isIntOrFloatType(const QByteArray &type)
 }
 
 WatchData::WatchData() :
-    id(0),
+    id(WatchData::InvalidId),
     state(InitialState),
     editformat(StopDisplay),
     editencoding(Unencoded8Bit),