Fix uninitialized values

Author: franzpoeschel

include/openPMD/IO/JSON/JSONIOHandlerImpl.hpp

@@ -307,8 +307,8 @@ class JSONIOHandlerImpl : public AbstractIOHandlerImpl
     {
         // Initialized in init()
         DatasetMode m_mode{};
-        SpecificationVia m_specificationVia;
-        bool m_skipWarnings;
+        SpecificationVia m_specificationVia = SpecificationVia::DefaultValue;
+        bool m_skipWarnings = false;
 
         template <typename A, typename B, typename C>
         operator std::tuple<A, B, C>()