Ticket #4192: fix crash if shadow is out of screen.

(tty_clip): add extra tests for area boundaries.

Signed-off-by: Andrew Borodin <aborodin@vmail.ru>

Author: Sergei Trofimovich

lib/tty/tty-ncurses.c

@@ -152,9 +152,16 @@ tty_clip (int *y, int *x, int *rows, int *cols)
 
     if (*y + *rows > LINES)
         *rows = LINES - *y;
+
+    if (*rows <= 0)
+        return FALSE;
+
     if (*x + *cols > COLS)
         *cols = COLS - *x;
 
+    if (*cols <= 0)
+        return FALSE;
+
     return TRUE;
 }