MINOR: logformat %st is signed

replace ultoa by ltoa for HTTP status code (can be -1)
diff --git a/src/log.c b/src/log.c
index e016e33..0b3be32 100644
--- a/src/log.c
+++ b/src/log.c
@@ -913,7 +913,7 @@
 				break;
 
 			case LOG_STATUS: // %st
-				tmplog = ultoa_o(txn->status, tmplog, MAX_SYSLOG_LEN - (tmplog - logline));
+				tmplog = ltoa_o(txn->status, tmplog, MAX_SYSLOG_LEN - (tmplog - logline));
 				if (!tmplog)
 					goto out;
 				last_isspace = 0;