cachedb_cassandra: fix NULL deref when cass_cluster_new() returns NULL

cass_cluster_new() can return NULL on allocation failure.  The existing code
has a NULL check, but it comes after cass_cluster_set_credentials() already
uses the pointer (when credentials are configured), so a NULL return causes
a crash before the check is reached.

Move the NULL check to immediately after cass_cluster_new(), before any use
of the returned pointer.

Found during a systematic audit of cachedb backends following the
cachedb_redis NULL-deref fix in commit 8fb569c.

(cherry picked from commit 8f959e7)

Author: dondetir

modules/cachedb_cassandra/cachedb_cassandra_dbase.c

@@ -141,13 +141,13 @@ int cassandra_reopen(cassandra_con *cass_con)
 int cassandra_new_connection(cassandra_con *con, char *host, int port, char *username, char *password)
 {
 	con->cluster = cass_cluster_new();
-	if (username && password) {
-	  cass_cluster_set_credentials(con->cluster, username, password);
-	}
 	if (!con->cluster) {
 		LM_ERR("Failed to create Cassandra Cluster object\n");
 		return -1;
 	}
+	if (username && password) {
+	  cass_cluster_set_credentials(con->cluster, username, password);
+	}
 
 #if CASS_VERSION_MAJOR >= 2 && CASS_VERSION_MINOR >= 15
 	/* since version 2.15, DSE support is available in the standard driver