Core/Misc: Fix GCC 8 warnings

(cherry picked from commit 1d04a3b216)

src/server/scripts/Kalimdor/zone_azuremyst_isle.cpp

@@ -563,7 +563,9 @@ public:
                     Spark->DisappearAndDie();
                     DespawnNagaFlag(false);
                     me->DisappearAndDie();
-                default: return 99999999;
+                    /* fallthrough */
+                default:
+                    return 99999999;
             }
         }
 

src/server/scripts/Northrend/IcecrownCitadel/boss_valithria_dreamwalker.cpp

@@ -231,7 +231,7 @@ class ValithriaDespawner : public BasicEvent
                 case NPC_VALITHRIA_DREAMWALKER:
                     if (InstanceScript* instance = creature->GetInstanceScript())
                         instance->SendEncounterUnit(ENCOUNTER_FRAME_DISENGAGE, creature);
-                    // no break
+                    /* fallthrough */
                 case NPC_BLAZING_SKELETON:
                 case NPC_SUPPRESSER:
                 case NPC_BLISTERING_ZOMBIE:

src/server/scripts/Northrend/zone_sholazar_basin.cpp

@@ -534,7 +534,7 @@ public:
                         break;
                     }
                 }
-                    /* fallthrough */
+                /* fallthrough */
                 case EVENT_MISS:
                 {
                     shooter->CastSpell(wilhelm, SPELL_MISS_APPLE);

src/server/scripts/Outland/HellfireCitadel/ShatteredHalls/shattered_halls.cpp

@@ -149,6 +149,8 @@ class boss_shattered_executioner : public CreatureScript
                             /* fallthrough */
                         case 1:
                             me->RemoveLootMode(LOOT_MODE_HARD_MODE_3);
+                            /* fallthrough */
+                        default:
                             break;
                     }
                 }