shared: check for valid UTF-8 in nm_utils_ifname_valid()

The interface name might come from the command line or from a filename
(like during nm_vpn_wireguard_import()). It's not clear that this
is valid UTF-8. Asserting against that requires the caller to ensure
that the encoding is valid. That is cumbersome, especially since we anyway
check. Just report a regular error.

shared/nm-glib-aux/nm-shared-utils.c

@@ -4127,7 +4127,11 @@ nm_utils_ifname_valid (const char* name,
 		return FALSE;
 	}
 
-	g_return_val_if_fail (g_utf8_validate (name, -1, NULL), FALSE);
+	if (!g_utf8_validate (name, -1, NULL)) {
+		g_set_error_literal (error, NM_UTILS_ERROR, NM_UTILS_ERROR_UNKNOWN,
+		                     _("interface name must be UTF-8 encoded"));
+		return FALSE;
+	}
 
 	switch (type) {
 	case NMU_IFACE_KERNEL: