dict.c: don't try buckets that are empty for sure in dictGetRandomKey().

This is very similar to the optimization applied to dictGetRandomKeys,
but applied to the single key variant.

Related to issue #2306.

src/dict.c

@@ -631,7 +631,11 @@ dictEntry *dictGetRandomKey(dict *d)
     if (dictIsRehashing(d)) _dictRehashStep(d);
     if (dictIsRehashing(d)) {
         do {
-            h = random() % (d->ht[0].size+d->ht[1].size);
+            /* We are sure there are no elements in indexes from 0
+             * to rehashidx-1 */
+            h = d->rehashidx + (random() % (d->ht[0].size +
+                                            d->ht[1].size -
+                                            d->rehashidx));
             he = (h >= d->ht[0].size) ? d->ht[1].table[h - d->ht[0].size] :
                                       d->ht[0].table[h];
         } while(he == NULL);