Don't SIGSEGV if no transport is used.

One of previous commits (74b1b62510d939) allowed us to connect to
localhost directly if ssh transport was used. However, if there's
not transport, we SIGSEGV'ed as g_str_equal doesn't like NULL as
one of arguments. Change this to g_strcmp0 which does the same
service but is more friendly to NULL arguments.

src/virt-viewer.c

@@ -376,7 +376,7 @@ virt_viewer_extract_connect_info(VirtViewer *self,
      */
     if (virt_viewer_replace_host(ghost)) {
         gchar *replacement_host = NULL;
-        if (g_str_equal(transport, "ssh")) {
+        if (g_strcmp0(transport, "ssh") == 0) {
             replacement_host = g_strdup("localhost");
         } else {
             replacement_host = g_strdup(host);