Checked for instant potions, to fix NPE in serialization. Fixes SPIGOT-749

By: olivervscreeper <dev@olivervscreeper.co.uk>

paper-api/src/main/java/org/bukkit/potion/Potion.java

@@ -405,7 +405,7 @@ public class Potion {
         if ((damage & SPLASH_BIT) > 0) {
             potion = potion.splash();
         }
-        if ((damage & EXTENDED_BIT) > 0) {
+        if ((!type.equals(PotionType.INSTANT_DAMAGE) || type.equals(PotionType.FIRE_RESISTANCE)) && (damage & EXTENDED_BIT) > 0) {
             potion = potion.extend();
         }
         return potion;