Fix CVE-2017-17969:

https://sourceforge.net/p/p7zip/bugs/204/
https://cve.mitre.org/cgi-bin/cvename.cgi?name=CVE-2017-17969

Patch copied from Debian.

Subject: Heap-based buffer overflow in 7zip/Compress/ShrinkDecoder.cpp
Origin: vendor, https://sourceforge.net/p/p7zip/bugs/_discuss/thread/0920f369/27d7/attachment/CVE-2017-17969.patch
Forwarded: https://sourceforge.net/p/p7zip/bugs/_discuss/thread/0920f369/#27d7
Bug: https://sourceforge.net/p/p7zip/bugs/204/
Bug-Debian: https://bugs.debian.org/888297
Bug-Debian-Security: https://security-tracker.debian.org/tracker/CVE-2017-17969
Author: Antoine Beaupré <anarcat@debian.org>
Reviewed-by: Salvatore Bonaccorso <carnil@debian.org>
Last-Update: 2018-02-01
Applied-Upstream: 18.00-beta

--- a/CPP/7zip/Compress/ShrinkDecoder.cpp
+++ b/CPP/7zip/Compress/ShrinkDecoder.cpp
@@ -121,8 +121,13 @@ HRESULT CDecoder::CodeReal(ISequentialIn
     {
       _stack[i++] = _suffixes[cur];
       cur = _parents[cur];
+      if (cur >= kNumItems || i >= kNumItems)
+        break;
     }
-    
+
+    if (cur >= kNumItems || i >= kNumItems)
+      break;
+
     _stack[i++] = (Byte)cur;
     lastChar2 = (Byte)cur;
 
d63545bf'>refs</a><a class='active' href='/guix/log/etc/historical-authorizations'>log</a><a href='/guix/tree/etc/historical-authorizations?id=e423df6fa61730103aa040a2f92097aed63545bf'>tree</a><a href='/guix/commit/etc/historical-authorizations?id=e423df6fa61730103aa040a2f92097aed63545bf'>commit</a><a href='/guix/diff/etc/historical-authorizations?id=e423df6fa61730103aa040a2f92097aed63545bf'>diff</a></td><td class='form'><form class='right' method='get' action='/guix/log/etc/historical-authorizations'>
<input type='hidden' name='id' value='e423df6fa61730103aa040a2f92097aed63545bf'/><select name='qt'>
<option value='grep'>log msg</option>
<option value='author'>author</option>
<option value='committer'>committer</option>
<option value='range'>range</option>
</select>
<input class='txt' type='search' size='10' name='q' value=''/>
<input type='submit' value='search'/>
</form>
</td></tr></table>
<div class='path'>path: <a href='/guix/log/?id=e423df6fa61730103aa040a2f92097aed63545bf'>root</a>/<a href='/guix/log/etc?id=e423df6fa61730103aa040a2f92097aed63545bf'>etc</a>/<a href='/guix/log/etc/historical-authorizations?id=e423df6fa61730103aa040a2f92097aed63545bf'>historical-authorizations</a></div><div class='content'><table class='list nowrap'><tr class='nohover'><th class='left'>Age</th><th class='left'>Commit message (<a href='/guix/log/etc/historical-authorizations?id=e423df6fa61730103aa040a2f92097aed63545bf&amp;showmsg=1'>Expand</a>)</th><th class='left'>Author</th></tr>
<tr><td><span title='2020-07-11 11:51:56 +0200'>2020-07-11</span></td><td><a href='/guix/commit/etc/historical-authorizations?id=4b9b8b2ec23e59af2275a6d2829e6e0f915b44b3'>Add 'etc/historical-authorizations'.</a><span class='msg-avail'>...</span></td><td>Ludovic Courtès</td></tr>