Check null before grabbing metadata owning plugin. Fixes BUKKIT-4665

MetadataStoreBase throws a NullPointerException when passed a null value
for setMetaData. The intended behavior is to throw an
IllegalArgumentException. This commit changes the value's null check to
occur before referencing the owning plugin of a value.

src/main/java/org/bukkit/metadata/MetadataStoreBase.java

@@ -25,8 +25,8 @@ public abstract class MetadataStoreBase<T> {
      * @throws IllegalArgumentException If value is null, or the owning plugin is null
      */
     public synchronized void setMetadata(T subject, String metadataKey, MetadataValue newMetadataValue) {
-        Plugin owningPlugin = newMetadataValue.getOwningPlugin();
         Validate.notNull(newMetadataValue, "Value cannot be null");
+        Plugin owningPlugin = newMetadataValue.getOwningPlugin();
         Validate.notNull(owningPlugin, "Plugin cannot be null");
         String key = disambiguate(subject, metadataKey);
         Map<Plugin, MetadataValue> entry = metadataMap.get(key);