saving msg history glitch (trunk only)

<Someone> discovered that it is possible to
have wins[WIN_MESSAGE] be null during
a hangup, not sure why yet.

Put a guard in to prevent de-referencing a null pointer.

win/tty/topl.c

@@ -469,6 +469,7 @@ boolean init;
 	register struct WinDesc *cw = wins[WIN_MESSAGE];
 	char *retstr = (char *)0;
 
+	if (!cw) return (char *)0;	/* bail */
 	/*
 	 * state 0 = normal return with string from msg history.
 	 * state 1 = finished with recall data, return toplines.