Avoid a null-pointer dereference which occurred when emulating DOS

interrupt 0x10 function 0x00 without dpy int tty.c being set.

PR:		35147
Submitted by:	Stefan Farfeleder <e0026813@stud3.tuwien.ac.at>

usr.bin/doscmd/int10.c

@@ -56,6 +56,8 @@ int10(regcontext_t *REGS)
 
 	switch (R_AH) {
 	case 0x00:		/* Set display mode */
+		if (!xmode)
+			goto unsupported;
 		init_mode(R_AL);
 		break;
 	case 0x01:		/* Define cursor */