Fix line 260: TypeError: imsg_user[last_user] is undefined

Not sure how Zharvek triggers this error using the module, but he managed to
do it twice, so add a bit of paranoia-check.

exec/sbbsimsg.js

@@ -255,7 +255,7 @@ while(bbs.online) {
 				break;
 			}
 			done=false;
-			while(bbs.online && !done && !console.aborted) {
+			while(bbs.online && !done && !console.aborted && imsg_user[last_user]) {
 				printf("\1[\1n\1h\x11\1n-[\1hQ\1nuit/\1hA\1nll]-\1h\x10 \1y%-25s \1c%s\1>"
 					,imsg_user[last_user].name,imsg_user[last_user].bbs);
 				switch(console.getkey(K_UPPER|K_NOECHO)) {