Avoid passing NULL pointers to curses functions.

This allows profanity to work without segfaulting from NULL
pointer dereferences when used with NetBSD libcurses.

Basic functionality was tested, there may be more NULL pointer
issues hiding.

src/ui/window.c

@@ -1863,7 +1863,12 @@ win_sub_print(WINDOW* win, char* msg, gboolean newline, gboolean wrap, int inden
 void
 win_sub_newline_lazy(WINDOW* win)
 {
-    int curx = getcurx(win);
+    int curx;
+
+    if (win == NULL) {
+        return;
+    }
+    curx = getcurx(win);
     if (curx > 0) {
         int cury = getcury(win);
         wmove(win, cury + 1, 0);