core: discard non UTF-8 search domains

Domains are exported via D-Bus and so they must be valid UTF-8.

RFC 1035 specifies that domain labels can contain any 8 bit values,
but also recommends that they follow the "preferred syntax" which only
allows letters, digits and hypens.

Don't introduce a strict validation of the preferred syntax, but at
least discard non UTF-8 search domains, as they will cause assertion
failures later when they are sent over D-Bus.

src/core/nm-l3-config-data.c

@@ -1419,6 +1419,9 @@ _check_and_add_domain(GPtrArray **p_arr, const char *domain)
     if (domain[0] == '.' || strstr(domain, ".."))
         return FALSE;
 
+    if (!g_utf8_validate(domain, -1, NULL))
+        return FALSE;
+
     len = strlen(domain);
     if (domain[len - 1] == '.') {
         copy   = g_strndup(domain, len - 1);