_cat/allocation: disk.avail can be zero, if so, want to show 100%

Closes #4835

src/main/java/org/elasticsearch/rest/action/cat/RestAllocationAction.java

@@ -153,7 +153,7 @@ public class RestAllocationAction extends AbstractCatAction {
             long avail = nodeStats.getFs().getTotal().getAvailable().bytes();
 
             short diskPercent = -1;
-            if (used >= 0 && avail > 0) {
+            if (used >= 0 && avail >= 0) {
                 diskPercent = (short) (used * 100 / (used + avail));
             }