input: discard any multibyte character when <Alt> is being held

Otherwise the <Alt> key would simply be ignored.  It's better to reject
the combination, to compel the user to be precise in what they type.

src/winio.c

@@ -938,6 +938,13 @@ int parse_kbinput(WINDOW *win)
 			else if (keycode == KEY_BACKSPACE || keycode == '\b' ||
 												keycode == DEL_CODE)
 				return CONTROL_SHIFT_DELETE;
+#endif
+#ifdef ENABLE_UTF8
+			else if (0xC0 <= keycode && keycode <= 0xFF && using_utf8()) {
+				while (key_buffer_len > 0 && 0x80 <= *key_buffer && *key_buffer <= 0xBF)
+					get_input(NULL);
+				return FOREIGN_SEQUENCE;
+			}
 #endif
 			else if (!solitary && keycode < 0x20)
 				meta_key = TRUE;