input: flush the keystroke buffer upon any kind of error condition

This stops the execution of a macro or a string bind whenever something
unexpected happens, to prevent the waiting keystrokes from doing things
that were not intended.

Especially this prevents an infinite loop: when during the recording
of a macro the `runmacro` keystroke is typed in some menu (where the
keystroke is not bound), and the macro is later replayed in a way that
results in exiting from that menu before the `runmacro` keystroke gets
replayed...

This fixes https://savannah.gnu.org/bugs/?65301.
The issue was reported by `correctmost`.

Bug existed since version 2.9.0, since the macro feature was introduced.

src/winio.c

@@ -2308,6 +2308,10 @@ void statusline(message_type importance, const char *msg, ...)
 	int colorpair;
 	va_list ap;
 
+	/* Drop all waiting keystrokes upon any kind of "error". */
+	if (importance >= AHEM)
+		waiting_codes = 0;
+
 	/* Ignore a message with an importance that is lower than the last one. */
 	if (importance < lastmessage && lastmessage > NOTICE)
 		return;