tagfile: Check if memchr() returned null before using

This fixes a segmentation fault trying to read from nullptr+1,
aka address 1.

1 files changed, 6 insertions, 1 deletions

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 "";