ProtobufDeserializer: fix uninitialized iterator member

Explicitly initialize m_it to avoid undefined behavior of uninitialized
class members.

Coverity-Id: 479445
Change-Id: I6af28d118ae12a25d58d697535da185c8e1c47e1
Reviewed-by: Alexey Edelev <alexey.edelev@qt.io>

src/protobuf/qprotobufserializer_p.h

@@ -74,7 +74,7 @@ public:
 
     void reset(QByteArrayView data);
 
-    QProtobufSelfcheckIterator m_it;
+    QProtobufSelfcheckIterator m_it = {};
 
 private:
     void setError(QAbstractProtobufSerializer::Error error, QAnyStringView errorString) override;