CLEANUP: http-ana: Remove useless test on txn when the error message is retrieved

In http_error_message(), the HTTP txn is always defined. So, this is no reason
to test its nullity.

This patch partially fixes the issue #457.
diff --git a/src/http_ana.c b/src/http_ana.c
index 82a9e14..23db04e 100644
--- a/src/http_ana.c
+++ b/src/http_ana.c
@@ -4603,7 +4603,7 @@
 {
 	const int msgnum = http_get_status_idx(s->txn->status);
 
-	if (s->txn && s->txn->errmsg)
+	if (s->txn->errmsg)
 		return s->txn->errmsg;
 	else if (s->be->errmsg[msgnum])
 		return s->be->errmsg[msgnum];