From 18be9305070a489036168f15b26a15e1ca520e79 Mon Sep 17 00:00:00 2001
From: Patrick Pelissier <patrick.pelissier@gmail.com>
Date: Wed, 18 Jul 2018 21:41:52 +0200
Subject: [PATCH] Fix bug in case of error returned by vsnprintf: the buffer
 may have been corrupted in case of error. Clean it before returning.

---
 m-string.h | 5 +++++
 1 file changed, 5 insertions(+)

diff --git a/m-string.h b/m-string.h
index 8ad61255..4038729d 100644
--- a/m-string.h
+++ b/m-string.h
@@ -629,9 +629,14 @@ string_cat_printf (string_t v, const char format[], ...)
     va_end (args);
     va_start (args, format);
     size = vsnprintf (&v->ptr[v->size], v->alloc - v->size, format, args);
+    assert (size >= 0);
   }
   if (size >= 0) {
     v->size += (size_t) size;
+  } else if (v->ptr != NULL) {
+    // vsnprintf may have output some characters before returning an error.
+    // Undo this to have a clean state
+    v->ptr[v->size] = 0;
   }
   va_end (args);
   STRINGI_CONTRACT (v);