fix segfault when mouse-moving multiple cards to empty tableu

fix segfault when mouse-moving multiple cards to empty tableu

the "up to?" dialog calls getch(NULL), which caused a segfault.
> Program received signal SIGSEGV, Segmentation fault.
> 0x00000000004030fb in getch (buf=0x0) at spider.c:940
> 940             if (buf[3]) {
> (gdb) bt
> #0  0x00000000004030fb in getch (buf=0x0) at spider.c:940
> #1  0x0000000000402c97 in get_cmd (from=0x7ffeba433bfc, to=0x7ffeba433bf8,
>     opt=0x7ffeba433bf4) at spider.c:808
> #2  0x0000000000400ebf in sol () at spider.c:108
> #3  0x0000000000400e2f in main (argc=2, argv=0x7ffeba433d28) at spider.c:88