samples: net: rpl_border_router: Fix out-of-bounds write

While printing debugging information, an out of bounds write could
happen while trying to write out the NUL byte in the url array.

Coverity-ID: 178790
Signed-off-by: Leandro Pereira <[email protected]>

Author: lpereira

samples/net/rpl_border_router/src/http.c

@@ -1528,7 +1528,7 @@ static void http_connected(struct http_ctx *ctx,
 			   void *user_data)
 {
 	char url[32];
-	int len = min(sizeof(url), ctx->http.url_len);
+	size_t len = min(sizeof(url) - 1, ctx->http.url_len);
 
 	NET_DBG("");