Index: chan_sip.c
===================================================================
--- chan_sip.c	(revision 29050)
+++ chan_sip.c	(working copy)
@@ -12121,10 +12121,12 @@
 			ast_log(LOG_WARNING, "Recv error: %s\n", strerror(errno));
 		return 1;
 	}
-	if (option_debug && res == sizeof(req.data))
+	if (option_debug && res == sizeof(req.data)) {
 		ast_log(LOG_DEBUG, "Received packet exceeds buffer. Data is possibly lost\n");
-
-	req.data[res] = '\0';
+		/* req.data[res] there would write after end of buffer */
+		req.data[sizeof(req.data)-1] = '\0';
+	} else
+		req.data[res] = '\0';
 	req.len = res;
 	if(sip_debug_test_addr(&sin))	/* Set the debug flag early on packet level */
 		ast_set_flag(&req, SIP_PKT_DEBUG);