prompt: at Yes-No, do not treat a screen resize as an invalid keystroke

Commit 12cf1c99 added a beep() for every invalid keystroke at the
Yes-No prompt, but overlooked that KEY_WINCH is not invalid.

This fixes https://savannah.gnu.org/bugs/?58422.

Bug existed since version 4.8, commit 12cf1c99.

src/prompt.c

@@ -678,6 +678,9 @@ int do_yesno_prompt(bool all, const char *msg)
 		kbinput = get_kbinput(bottomwin, !all);
 
 #ifndef NANO_TINY
+		if (kbinput == KEY_WINCH)
+			continue;
+
 		/* Accept the first character of an external paste. */
 		if (bracketed_paste && kbinput == BRACKETED_PASTE_MARKER)
 			kbinput = get_kbinput(bottomwin, BLIND);