cachedb_redis: fix NULL deref when redisConnect returns NULL

redisConnect() and redisConnectWithTimeout() can return NULL on allocation
failure.  The existing check only handles ctx->err != REDIS_OK (non-NULL
ctx with error), so a NULL return falls through to redisSetTimeout(NULL, ...)
which dereferences the NULL pointer.

Add an explicit NULL check after the error-code check.

Also change the warned flag from char to int to avoid undefined behavior
on signed overflow after 127 connection-timeout warnings.

Author: NormB

modules/cachedb_redis/cachedb_redis_dbase.c

@@ -58,7 +58,7 @@ static unsigned int redis_calc_escaped_len_json(str *s);
 redisContext *redis_get_ctx(char *ip, int port)
 {
 	struct timeval tv;
-	static char warned = 0;
+	static int warned = 0;
 	redisContext *ctx;
 
 	if (!port)
@@ -79,6 +79,12 @@ redisContext *redis_get_ctx(char *ip, int port)
 		return NULL;
 	}
 
+	if (!ctx) {
+		LM_ERR("failed to connect to redis %s:%hu - out of memory\n",
+				ip, (unsigned short)port);
+		return NULL;
+	}
+
 	if (redis_query_tout) {
 		tv.tv_sec = redis_query_tout / 1000;
 		tv.tv_usec = (redis_query_tout * 1000) % 1000000;