Fix memory leak in ZDIFF algorithm 2 on early exit (#14932)

## Problem

`zdiffAlgorithm2()` can break out early once the destination cardinality
reaches zero. In that path, a temporary SDS created by
`zuiSdsFromValue()` is left dirty and never released, because the
cleanup normally happens on the next `zuiNext()` call which is skipped
due to the early `break`.

`zuiClearIterator()` called after the loop does **not** clean up dirty
values — only `zuiNext()` or explicit `zuiDiscardDirtyValue()` does.

## Fix

Add `zuiDiscardDirtyValue(&zval)` before the early `break` to ensure the
temporary SDS is freed on all exit paths.

src/t_zset.c

@@ -2793,7 +2793,10 @@ static void zdiffAlgorithm2(zsetopsrc *src, long setnum, zset *dstzset, size_t *
 
             /* Exit if result set is empty as any additional removal
                 * of elements will have no effect. */
-            if (cardinality == 0) break;
+            if (cardinality == 0) {
+                zuiDiscardDirtyValue(&zval);
+                break;
+            }
         }
         zuiClearIterator(&src[j]);
 

tests/unit/type/zset.tcl

@@ -1114,6 +1114,18 @@ start_server {tags {"zset"}} {
             assert_equal {a 1 e 5} [r zrange zsete{t} 0 -1 withscores]
         }
 
+        test "ZDIFF algorithm 2 empty result early exit - $encoding" {
+            # Force algorithm 2 by inflating setnum with non-existing keys.
+            # algo_one_work = len(src[0]) * setnum / 2 = 2 * 10 / 2 = 10
+            # algo_two_work = 2 + 2 + 0*8 = 4
+            # algo_one (10) > algo_two (4) -> algorithm 2 is selected
+            r del zseta{t} zsetb{t} zsetc{t}
+            r zadd zseta{t} 1 a 2 b
+            r zadd zsetb{t} 1 a 2 b
+            assert_equal 0 [r zdiffstore zsetc{t} 10 zseta{t} zsetb{t} nx1{t} nx2{t} nx3{t} nx4{t} nx5{t} nx6{t} nx7{t} nx8{t}]
+            assert_equal {} [r zrange zsetc{t} 0 -1 withscores]
+        }
+
         test "ZDIFF fuzzing - $encoding" {
             for {set j 0} {$j < 100} {incr j} {
                 unset -nocomplain s