cfgutils: init probability pointer to NULL

- otherwise it can be an attempt to free it when the kamailio does not
  start due to config errors
- reported by Dragos Oancea

modules/cfgutils/cfgutils.c

@@ -131,7 +131,7 @@ static int mod_init(void);
 static void mod_destroy(void);
 
 static int initial_prob = 10;
-static int *probability;
+static int *probability = NULL;
 
 static char config_hash[MD5_LEN];
 static char* hash_file = NULL;