fix hang with preview when droppping to shell

managed to run into it a couple times not fully reproducable but if the
preview takes a bit to load and you drop to shell in that time it'll do
weird things. best to make sure to cancel it like it does in many other
scenarios

fuf.c

@@ -840,6 +840,7 @@ main(int argc, char *argv[])
 				}
 				break;
 			case 't':
+				cancel_preview();
 				sendwin();
 				setenv("FUFSEL", items[sel_item].name, 1);
 				open_with(getenv("SHELL"), NULL, true);