Core/Misc: Fix some static analysis issues

Add some asserts and additional NULL checks as sanity checks.

src/server/game/Entities/Player/Player.cpp

@@ -4205,6 +4205,7 @@ bool Player::Has310Flyer(bool checkAllSpells, uint32 excludeSpellId)
                     break;  // We can break because mount spells belong only to one skillline (at least 310 flyers do)
 
                 spellInfo = sSpellMgr->GetSpellInfo(itr->first);
+                ASSERT(spellInfo);
                 for (uint8 i = 0; i < MAX_SPELL_EFFECTS; ++i)
                     if (spellInfo->Effects[i].ApplyAuraName == SPELL_AURA_MOD_INCREASE_MOUNTED_FLIGHT_SPEED &&
                         spellInfo->Effects[i].CalcValue() == 310)
@@ -15743,6 +15744,7 @@ bool Player::SatisfyQuestExclusiveGroup(Quest const* qInfo, bool msg)
 
         // not allow have daily quest if daily quest from exclusive group already recently completed
         Quest const* Nquest = sObjectMgr->GetQuestTemplate(exclude_Id);
+        ASSERT(Nquest);
         if (!SatisfyQuestDay(Nquest, false) || !SatisfyQuestWeek(Nquest, false) || !SatisfyQuestSeasonal(Nquest, false))
         {
             if (msg)
@@ -15944,6 +15946,7 @@ bool Player::TakeQuestSourceItem(uint32 questId, bool msg)
                 return false;
             }
 
+            ASSERT(item);
             bool destroyItem = true;
             for (uint8 n = 0; n < QUEST_ITEM_OBJECTIVES_COUNT; ++n)
                 if (item->StartQuest == questId && srcItemId == quest->RequiredItemId[n])
@@ -21330,6 +21333,7 @@ bool Player::ActivateTaxiPathTo(std::vector<uint32> const& nodes, Creature* npc
     if (sWorld->getBoolConfig(CONFIG_INSTANT_TAXI))
     {
         TaxiNodesEntry const* lastPathNode = sTaxiNodesStore.LookupEntry(nodes[nodes.size()-1]);
+        ASSERT(lastPathNode);
         m_taxi.ClearTaxiDestinations();
         TeleportTo(lastPathNode->map_id, lastPathNode->x, lastPathNode->y, lastPathNode->z, GetOrientation());
         return false;
@@ -21550,6 +21554,7 @@ inline bool Player::_StoreOrEquipNewItem(uint32 vendorslot, uint32 item, uint8 c
     if (crItem->ExtendedCost)                            // case for new honor system
     {
         ItemExtendedCostEntry const* iece = sItemExtendedCostStore.LookupEntry(crItem->ExtendedCost);
+        ASSERT(iece);
         if (iece->reqhonorpoints)
             ModifyHonorPoints(- int32(iece->reqhonorpoints * count));