MINOR: debug: make ha_panic() report threads starting at 1

Internally they start at zero but everywhere (config, dumps) we show
them starting at 1, so let's fix the confusion.
diff --git a/src/debug.c b/src/debug.c
index b4fcb65..ecb55f5 100644
--- a/src/debug.c
+++ b/src/debug.c
@@ -141,7 +141,7 @@
 void ha_panic()
 {
 	chunk_reset(&trash);
-	chunk_appendf(&trash, "Thread %u is about to kill the process.\n", tid);
+	chunk_appendf(&trash, "Thread %u is about to kill the process.\n", tid + 1);
 	ha_thread_dump_all_to_trash();
 	write(2, trash.area, trash.data);
 	for (;;)