Avoid compiler warning about missing 'return' statement

dune/elasticity/materials/mooneyrivlinenergy.hh

@@ -181,10 +181,10 @@ energy(const Entity& element,
     return strainEnergyWithLog;
   } else if (mooneyrivlin_energy == "square") {
     return strainEnergyWithSquare;
-  } else {
-    return strainEnergyCiarlet;
   }
   std::cout << std::fixed;
+
+  return strainEnergyCiarlet;
 }
 
 }  // namespace Dune