startup_tcb: move get_scno invocation to a more convenient place

* strace.c (trace): Move get_scno invocation ...
(startup_tcb): ... here.  This is a no-op change that slightly improves
readability.

strace.c

@@ -2181,6 +2181,9 @@ startup_tcb(struct tcb *tcp)
 			}
 		}
 	}
+
+	if (get_scno(tcp) == 1)
+		tcp->s_prev_ent = tcp->s_ent;
 }
 
 static void
@@ -2363,8 +2366,6 @@ trace(void)
 	/* Is this the very first time we see this tracee stopped? */
 	if (tcp->flags & TCB_STARTUP) {
 		startup_tcb(tcp);
-		if (get_scno(tcp) == 1)
-			tcp->s_prev_ent = tcp->s_ent;
 	}
 
 	sig = WSTOPSIG(status);