input: prevent 'macro_length' from underflowing when hammering M-:

Normally, when recording a macro, users will make their keystrokes
slowly and carefully, and will most likely wait to see the effect
of the previous keystroke before making the next.  So, the chances
of two `recordmacro` keystrokes coming in in quick succession is
normally nil.  The 'macro_length' variable just needs a guard to
prevent it from underflowing when someone is hammering the keys.

This fixes https://savannah.gnu.org/bugs/?65394 in a better way.

src/winio.c

@@ -91,6 +91,7 @@ void add_to_macrobuffer(int code)
 /* Remove the last key code plus any leading Esc codes from macro buffer. */
 void snip_last_keystroke(void)
 {
+	if (macro_length > 0)
 		macro_length--;
 	while (macro_length > 0 && macro_buffer[macro_length - 1] == '\x1b')
 		macro_length--;