Fix cluster reset memory leak

[maybe] Fixes valgrind errors:
32 bytes in 4 blocks are definitely lost in loss record 107 of 228
   at 0x80EA447: je_malloc (jemalloc.c:944)
   by 0x806E59C: zrealloc (zmalloc.c:125)
   by 0x80A9AFC: clusterSetMaster (cluster.c:801)
   by 0x80AEDC9: clusterCommand (cluster.c:3994)
   by 0x80682A5: call (redis.c:2049)
   by 0x8068A20: processCommand (redis.c:2309)
   by 0x8076497: processInputBuffer (networking.c:1143)
   by 0x8073BAF: readQueryFromClient (networking.c:1208)
   by 0x8060E98: aeProcessEvents (ae.c:412)
   by 0x806123B: aeMain (ae.c:455)
   by 0x806C3DB: main (redis.c:3832)

64 bytes in 8 blocks are definitely lost in loss record 143 of 228
   at 0x80EA447: je_malloc (jemalloc.c:944)
   by 0x806E59C: zrealloc (zmalloc.c:125)
   by 0x80AAB40: clusterProcessPacket (cluster.c:801)
   by 0x80A847F: clusterReadHandler (cluster.c:1975)
   by 0x30000FF: ???

80 bytes in 10 blocks are definitely lost in loss record 148 of 228
   at 0x80EA447: je_malloc (jemalloc.c:944)
   by 0x806E59C: zrealloc (zmalloc.c:125)
   by 0x80AAB40: clusterProcessPacket (cluster.c:801)
   by 0x80A847F: clusterReadHandler (cluster.c:1975)
   by 0x2FFFFFF: ???

src/cluster.c

@@ -842,6 +842,7 @@ void freeClusterNode(clusterNode *n) {
     /* Release link and associated data structures. */
     if (n->link) freeClusterLink(n->link);
     listRelease(n->fail_reports);
+    zfree(n->slaves);
     zfree(n);
 }