strace.c: reset printing_tcp on print_event_exit

print_event_exit should have printing_tcp setup analogous to printleader and
syscall_exit_trace before printing its part, otherwise it spuriously resets
curcol in line_ended for the stale printing_tcp if followfork == 2.

* strace.c (print_event_exit): Set printing_tcp to tcp before printing
exiting event line and calling line_ended.

strace.c

@@ -2232,6 +2232,8 @@ print_event_exit(struct tcb *tcp)
 		 */
 		tprints(" <unfinished ...>");
 	}
+
+	printing_tcp = tcp;
 	tprints(") ");
 	tabto();
 	tprints("= ?\n");