Fix gcc8 catch-value "catch expection by value" warnings

Author: nyalldawson

src/core/pal/pal.cpp

@@ -468,7 +468,7 @@ QList<LabelPosition *> Pal::solveProblem( Problem *prob, bool displayAll )
     else
       prob->popmusic();
   }
-  catch ( InternalException::Empty )
+  catch ( InternalException::Empty & )
   {
     return QList<LabelPosition *>();
   }

src/core/pal/problem.cpp

@@ -290,7 +290,7 @@ void Problem::init_sol_falp()
       {
         list->insert( label, mLabelPositions.at( label )->getNumOverlaps() );
       }
-      catch ( pal::InternalException::Full )
+      catch ( pal::InternalException::Full & )
       {
         continue;
       }