Fix potential crash originating from HookSpellCast (#822)

Author: miredirex

Code/client/Games/Skyrim/Magic/ActorMagicCaster.cpp

@@ -18,9 +18,13 @@ void TP_MAKE_THISCALL(HookSpellCast, ActorMagicCaster, bool abSuccess, int32_t a
 {
     spdlog::debug("HookSpellCast, abSuccess: {}, auiTargetCount: {}, apSpell: {:X}", abSuccess, auiTargetCount, (uint64_t)apSpell);
 
-    // TODO(cosideci): why is this here?
+    // Note: these if guards is how the game does it too
+    if (!apThis->pCasterActor)
+        return;
     if (!abSuccess)
         return;
+    if (!apSpell && !apThis->pCurrentSpell)
+        return;
 
     if (apThis->pCasterActor->GetExtension()->IsRemote())
         return;

Code/client/Games/Skyrim/Magic/ActorMagicCaster.h

@@ -28,4 +28,6 @@ struct ActorMagicCaster : MagicCaster
     uint32_t uiFlags;
 };
 
+static_assert(offsetof(ActorMagicCaster, pCasterActor) == 0xB8);
+static_assert(offsetof(ActorMagicCaster, pMagicNode) == 0xC0);
 static_assert(sizeof(ActorMagicCaster) == 0x100);