6875393: 2/3 JNI itable index cache is broken

Add missing initialization of cache size.

Reviewed-by: tbell

hotspot/src/share/vm/oops/instanceKlass.cpp

@@ -1073,6 +1073,7 @@ void instanceKlass::set_cached_itable_index(size_t idnum, int index) {
     if (indices == NULL || (length = (size_t)indices[0]) <= idnum) {
       size_t size = MAX2(idnum+1, (size_t)idnum_allocated_count());
       int* new_indices = NEW_C_HEAP_ARRAY(int, size+1);
+      new_indices[0] =(int)size;  // array size held in the first element
       // Copy the existing entries, if any
       size_t i;
       for (i = 0; i < length; i++) {