url: silence MSVC warning

Since commit f3d501dc67, if proxy support is disabled, MSVC warns:
url.c : warning C4701: potentially uninitialized local variable
'hostaddr' used
url.c : error C4703: potentially uninitialized local pointer variable
'hostaddr' used

That could actually only happen if both `conn->bits.proxy` and
`CURL_DISABLE_PROXY` were enabled.
Initialize it to NULL to silence the warning.

Closes https://github.com/curl/curl/pull/5638

lib/url.c

@@ -3170,7 +3170,7 @@ static CURLcode resolve_server(struct Curl_easy *data,
   else {
     /* this is a fresh connect */
     int rc;
-    struct Curl_dns_entry *hostaddr;
+    struct Curl_dns_entry *hostaddr = NULL;
 
 #ifdef USE_UNIX_SOCKETS
     if(conn->unix_domain_socket) {