Fixed several warnings with fallthrough on gcc 8

src/OpenMesh/Apps/Subdivider/adaptive_subdivider.cc

@@ -170,7 +170,7 @@ int main(int argc, char **argv)
       case 'm': { std::stringstream s; s << optarg; s >> max_nv; } break;
       case 'r': rule_sequence = optarg; break;
       case 'U': uniform = true; break;
-      case 'h': usage_and_exit(argv[0],0);
+      case 'h': usage_and_exit(argv[0],0); break;
       case '?':
       default:  usage_and_exit(argv[0],1);
     }

src/OpenMesh/Apps/Subdivider/subdivider.cc

@@ -234,7 +234,7 @@ int main(int argc, char **argv)
         }
         break;
       }
-      case 'h': usage_and_exit(0);
+      case 'h': usage_and_exit(0); break;
       case '?':
       default:  usage_and_exit(1);
     }