Fixed pattern "X%Y" with no Y defined throwing an exception.

Fixes WORLDEDIT-2897.

src/main/java/com/sk89q/worldedit/WorldEdit.java

@@ -713,8 +713,12 @@ public class WorldEdit {
             // Parse special percentage syntax
             if (s.matches("[0-9]+(\\.[0-9]*)?%.*")) {
                 String[] p = s.split("%");
+                if (p.length < 2) {
+                    throw new UnknownItemException(s);
+                } else {
                     chance = Double.parseDouble(p[0]);
                     block = getBlock(player, p[1]);
+                }
             } else {
                 chance = 1;
                 block = getBlock(player, s);