Add resistance to MultiItems to prevent crashes from invalid stacks.

items/materials/ItemMultiMaterial.java

@@ -166,7 +166,7 @@ public class ItemMultiMaterial extends AEBaseItem implements IStorageComponent,
 	{
 		if ( dmgToMaterial.containsKey( is.getItemDamage() ) )
 			return dmgToMaterial.get( is.getItemDamage() );
-		return null;
+		return MaterialType.InvalidType;
 	}
 
 	@Override

items/materials/MaterialType.java

@@ -17,6 +17,8 @@ import cpw.mods.fml.relauncher.SideOnly;
 
 public enum MaterialType
 {
+	InvalidType(-1, AEFeature.Core),
+
 	CertusQuartzCrystal(0, AEFeature.Core, "crystalCertusQuartz"), CertusQuartzCrystalCharged(1, AEFeature.Core, EntityChargedQuartz.class),
 
 	CertusQuartzDust(2, AEFeature.Core, "dustCertusQuartz"), NetherQuartzDust(3, AEFeature.Core, "dustNetherQuartz"), Flour(4, AEFeature.Flour, "dustWheat"), GoldDust(