Fix unexpected exception when downloading reference peak CSV.

a std::function was being converted to a boost::function, which when the std::function was nullptr, is was creating a boost::function that tested non-empty.

src/ReferencePhotopeakDisplay.cpp

@@ -278,7 +278,7 @@ namespace
       }//switch( refinfo.m_source_type )
 
 
-      boost::function<double(float)> att_fcn = refinfo.m_input.m_shielding_att;
+      std::function<double( float )> att_fcn = refinfo.m_input.m_shielding_att;
 
       try
       {
@@ -318,7 +318,9 @@ namespace
           assert( refinfo.m_input.m_shielding_an.empty() );
           assert( refinfo.m_input.m_shielding_ad.empty() );
           assert( refinfo.m_input.m_shielding_name.empty() );
-          assert( refinfo.m_input.m_shielding_thickness.empty() );
+
+          // We may have a shielding thickness, but no material
+          //assert( refinfo.m_input.m_shielding_thickness.empty() );
 
           out << "Shielding,None" << eol_char;
         }//