[jvm] Fix off-by-one in multi cache

Author: usev6

src/vm/jvm/runtime/org/perl6/nqp/sixmodel/reprs/MultiCacheInstance.java

@@ -96,9 +96,12 @@ public void add(CallCaptureInstance capture, SixModelObject result, ThreadContex
         if (numArgs >= MD_CACHE_MAX_ARITY)
             return;
 
+        /* The zero arity case was handled above.
+         * At index 0 we have the cache for arity 1 */
+        ArityCache ac = this.arityCaches[numArgs - 1];
+
         /* If the cache is saturated, don't do anything (we could instead do a random
          * replacement). */
-        ArityCache ac = this.arityCaches[numArgs];
         if (ac != null && ac.numEntries == MD_CACHE_MAX_ENTRIES)
             return;