Update ziplist.c

Hi, @antirez

In the code, to get the size of ziplist, "unsigned int bytes = ZIPLIST_HEADER_SIZE+1;" is correct, 
but why not make it more readable and easy to understand

src/ziplist.c

@@ -576,7 +576,7 @@ void zipEntry(unsigned char *p, zlentry *e) {
 
 /* Create a new empty ziplist. */
 unsigned char *ziplistNew(void) {
-    unsigned int bytes = ZIPLIST_HEADER_SIZE+1;
+    unsigned int bytes = ZIPLIST_HEADER_SIZE+ZIPLIST_END_SIZE;
     unsigned char *zl = zmalloc(bytes);
     ZIPLIST_BYTES(zl) = intrev32ifbe(bytes);
     ZIPLIST_TAIL_OFFSET(zl) = intrev32ifbe(ZIPLIST_HEADER_SIZE);