input: don't crash when receiving a KEY_CANCEL

On some systems, typing ^C apparently results in a KEY_CANCEL in
the input stream, which gets hard-bound to the do_cancel function.
But in the main menu there is no Cancel function.  So... in that
case, let it fall back to the plain old Ctrl-C code: 0x03.

Reported-by: Liam Gretton <liam.gretton@leicester.ac.uk>

src/winio.c

@@ -667,7 +667,7 @@ int parse_kbinput(WINDOW *win)
 #endif
 	/* Slang doesn't support KEY_CANCEL. */
 	case KEY_CANCEL:
-	    return first_sc_for(currmenu, do_cancel)->keycode;
+	    return sc_seq_or(do_cancel, 0x03);
 #endif
 #ifdef KEY_SUSPEND
 #ifdef KEY_SSUSPEND