Don't check magicka when casting free spells

CHANGELOG.md

@@ -98,6 +98,7 @@
     Bug #5758: Paralyzed actors behavior is inconsistent with vanilla
     Bug #5762: Movement solver is insufficiently robust
     Bug #5821: NPCs from mods getting removed if mod order was changed
+    Bug #5841: Can't Cast Zero Cost Spells When Magicka is < 0
     Feature #390: 3rd person look "over the shoulder"
     Feature #1536: Show more information about level on menu
     Feature #2386: Distant Statics in the form of Object Paging

apps/openmw/mwmechanics/spellutil.cpp

@@ -123,7 +123,7 @@ namespace MWMechanics
         if (spell->mData.mType != ESM::Spell::ST_Spell)
             return 100;
 
-        if (checkMagicka && stats.getMagicka().getCurrent() < spell->mData.mCost)
+        if (checkMagicka && spell->mData.mCost > 0 && stats.getMagicka().getCurrent() < spell->mData.mCost)
             return 0;
 
         if (spell->mData.mFlags & ESM::Spell::F_Always)

apps/openmw/mwworld/worldimp.cpp

@@ -3009,7 +3009,7 @@ namespace MWWorld
             // Check mana
             bool godmode = (isPlayer && mGodMode);
             MWMechanics::DynamicStat<float> magicka = stats.getMagicka();
-            if (magicka.getCurrent() < spell->mData.mCost && !godmode)
+            if (spell->mData.mCost > 0 && magicka.getCurrent() < spell->mData.mCost && !godmode)
             {
                 message = "#{sMagicInsufficientSP}";
                 fail = true;