Core/Spell: fix a potential null dereference crash

src/server/game/Spells/SpellInfo.cpp

@@ -1422,7 +1422,7 @@ bool SpellInfo::IsAffectedBySpellMod(SpellModifier const* mod) const
 bool SpellInfo::CanPierceImmuneAura(SpellInfo const* auraSpellInfo) const
 {
     // aura can't be pierced
-    if (auraSpellInfo->HasAttribute(SPELL_ATTR0_UNAFFECTED_BY_INVULNERABILITY))
+    if (!auraSpellInfo || auraSpellInfo->HasAttribute(SPELL_ATTR0_UNAFFECTED_BY_INVULNERABILITY))
         return false;
 
     // these spells pierce all available spells (Resurrection Sickness for example)