Add macro to define clusterNode.ip buffer size.

Add REDIS_CLUSTER_IPLEN macro to define the size of the clusterNode ip
character array. Additionally use this macro in inet_ntop(3) calls where
the size of the array was being defined manually.

The REDIS_CLUSTER_IPLEN is defined as INET_ADDRSTRLEN which defines the
correct size of a buffer to store an IPv4 address in. The
INET_ADDRSTRLEN macro itself is defined in the <netinet/in.h> header
file and should be portable across the majority of systems.

src/redis.h

@@ -120,6 +120,7 @@
 #define REDIS_DEFAULT_AOF_REWRITE_INCREMENTAL_FSYNC 1
 #define REDIS_DEFAULT_MIN_SLAVES_TO_WRITE 0
 #define REDIS_DEFAULT_MIN_SLAVES_MAX_LAG 10
+#define REDIS_IP_STR_LEN INET6_ADDRSTRLEN
 #define REDIS_BINDADDR_MAX 16
 
 /* Protocol and I/O related defines */