From 942be407ee8b6ca1089ed9c2f135ca4ed89c44fc Mon Sep 17 00:00:00 2001
From: Julian Andres Klode <julian.klode@canonical.com>
Date: Thu, 20 Feb 2020 13:25:10 +0100
Subject: tagfile: Check if memchr() returned null before using

This fixes a segmentation fault trying to read from nullptr+1,
aka address 1.
---
 apt-pkg/tagfile.cc | 7 ++++++-
 1 file changed, 6 insertions(+), 1 deletion(-)

diff --git a/apt-pkg/tagfile.cc b/apt-pkg/tagfile.cc
index bbece1d7e..b86936353 100644
--- a/apt-pkg/tagfile.cc
+++ b/apt-pkg/tagfile.cc
@@ -714,8 +714,13 @@ StringView pkgTagSection::Find(Key key) const
 StringView pkgTagSection::FindRawInternal(unsigned int Pos) const
 {
    char const *Start = (char const *) memchr(Section + d->Tags[Pos].EndTag, ':', d->Tags[Pos].StartValue - d->Tags[Pos].EndTag);
-   ++Start;
    char const *End = Section + d->Tags[Pos + 1].StartTag;
+
+   if (Start == nullptr)
+      return "";
+
+   ++Start;
+
    if (unlikely(Start > End))
       return "";
 
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