startup: don't call delwin() with NULL, to avoid crashing on Solaris

Apparently the curses on SunOS is less forgiving than the one on GNU.
Or rather: delwin(NULL) should just return an error, it shouldn't crash.

This fixes https://savannah.gnu.org/bugs/?51053

.
Reported-by: John Wiersba <jrw32982@yahoo.com>
Solved-by: John Wiersba <jrw32982@yahoo.com>

src/global.c

@@ -95,14 +95,14 @@ char *present_path = NULL;
 
 unsigned flags[4] = {0, 0, 0, 0};
 	/* Our flag containing the states of all global options. */
-WINDOW *topwin;
+WINDOW *topwin = NULL;
 	/* The top portion of the window, where we display the version
 	 * number of nano, the name of the current file, and whether the
 	 * current file has been modified. */
-WINDOW *edit;
+WINDOW *edit = NULL;
 	/* The middle portion of the window, i.e. the edit window, where
 	 * we display the current file we're editing. */
-WINDOW *bottomwin;
+WINDOW *bottomwin = NULL;
 	/* The bottom portion of the window, where we display statusbar
 	 * messages, the statusbar prompt, and a list of shortcuts. */
 int editwinrows = 0;
@@ -1728,7 +1728,8 @@ int strtomenu(const char *input)
  * function unless debugging is turned on. */
 void thanks_for_all_the_fish(void)
 {
-    delwin(topwin);
+    if (topwin != NULL)
+	delwin(topwin);
     delwin(edit);
     delwin(bottomwin);
 

src/nano.c

@@ -677,14 +677,17 @@ void die_save_file(const char *die_filename, struct stat *die_stat)
 /* Initialize the three window portions nano uses. */
 void window_init(void)
 {
-    /* First delete existing windows, in case of resizing. */
-    delwin(topwin);
-    topwin = NULL;
-    delwin(edit);
-    delwin(bottomwin);
+    /* When resizing, first delete the existing windows. */
+    if (edit != NULL) {
+	if (topwin != NULL)
+	    delwin(topwin);
+	delwin(edit);
+	delwin(bottomwin);
+    }
 
     /* If the terminal is very flat, don't set up a titlebar. */
     if (LINES < 3) {
+	topwin = NULL;
 	editwinrows = 1;
 	/* Set up two subwindows.  If the terminal is just one line,
 	 * edit window and statusbar window will cover each other. */