multiple calls of newterm can return the same SCREEN *

[Mike Gran]

Hello.

I was checking out the multiple screen functionality
ncurses when writing a unit test, and I'm not sure that
it works as described in the docs.

Granted, I'd be very unlikely to use multiple screens 
outside of unit testing, but, anyway...

In the example below, I've created screens from two different
files, but the return from newterm() is the same for both.


On my box, the output is 


version: ncurses 6.0.20160116
screen #1: 0x00602470   stdscr #1: 0x0066ca10
screen #2: 0x00602470   stdscr #2: 0x0066ca10


---
#include <stdio.h>
#include <string.h>
#include <stdlib.h>
#include <ncursesw/curses.h>

int main()
{
FILE *fp1 = fopen("screen1.dat", "w+");
FILE *fp2 = fopen("screen2.dat", "w+");
SCREEN *screen1 = newterm ("vt100", fp1, fp1);
SCREEN *screen2 = newterm ("xterm", fp2, fp2);
WINDOW *stdscr1, *stdscr2;

set_term(screen1);
stdscr1 = stdscr;
addstr("hello #1");
refresh();
endwin();

set_term(screen2);
stdscr2 = stdscr;
addstr("hello #2");
refresh();
// endwin();  <-- crashes here

printf("version: %s\n", curses_version());
printf("screen #1: 0x%08x   stdscr #1: 0x%08x\n", screen1, stdscr1);
printf("screen #2: 0x%08x   stdscr #2: 0x%08x\n", screen2, stdscr2);

delscreen(screen2);
// delscreen(screen1);  <-- double free here

fclose (fp2);
fclose (fp1);
return 0;
}