Fix cluster migrate memory leak

Fixes valgrind error:
48 bytes in 1 blocks are definitely lost in loss record 196 of 373
   at 0x4910D3: je_malloc (jemalloc.c:944)
   by 0x42807D: zmalloc (zmalloc.c:125)
   by 0x41FA0D: dictGetIterator (dict.c:543)
   by 0x41FA48: dictGetSafeIterator (dict.c:555)
   by 0x459B73: clusterHandleSlaveMigration (cluster.c:2776)
   by 0x45BF27: clusterCron (cluster.c:3123)
   by 0x423344: serverCron (redis.c:1239)
   by 0x41D6CD: aeProcessEvents (ae.c:311)
   by 0x41D8EA: aeMain (ae.c:455)
   by 0x41A84B: main (redis.c:3832)

Author: mattsta

src/cluster.c

@@ -2803,6 +2803,7 @@ void clusterHandleSlaveMigration(int max_slaves) {
             }
         }
     }
+    dictReleaseIterator(di);
 
     /* Step 4: perform the migration if there is a target, and if I'm the
      * candidate. */