Fix repeated iterator generation (apache#10722)

### Motivation
Fix repeated iterator generation

(cherry picked from commit e678024)

pulsar-common/src/main/java/org/apache/pulsar/common/util/collections/LongPairRangeSet.java

@@ -274,7 +274,7 @@ public boolean contains(long key, long value) {
         public Range<T> firstRange() {
             Iterator<Range<T>> iterable = set.asRanges().iterator();
             if (iterable.hasNext()) {
-                return set.asRanges().iterator().next();
+                return iterable.next();
             }
             return null;
         }