Check that error exists before trying to set it. Fixes bug #640975.

glib/gconvert.c

@@ -769,6 +769,7 @@ g_convert_with_iconv (const gchar *str,
 	      have_error = TRUE;
 	      break;
 	    default:
+	      if (error)
                 {
                   int errsv = errno;