p7zip: Fix CVE-2017-17969

[No upstream tracking] -- https://bugs.debian.org/cgi-bin/bugreport.cgi?bug=888297

Heap-based buffer overflow in 7zip

Compress/ShrinkDecoder.cpp: Heap-based buffer overflow
in the NCompress::NShrink::CDecoder::CodeReal method
in 7-Zip before 18.00 and p7zip allows remote attackers
to cause a denial of service (out-of-bounds write)
or potentially execute arbitrary code via a crafted ZIP archive. 

Upstream-Status: Backport [https://sourceforge.net/p/p7zip/bugs/_discuss/thread/0920f369/8316/attachment/CVE-2017-17969.patch]
CVE: CVE-2017-17969
Signed-off-by: Radovan Scasny <radovan.scasny@siemens.com>

--- a/CPP/7zip/Compress/ShrinkDecoder.cpp
+++ b/CPP/7zip/Compress/ShrinkDecoder.cpp
@@ -121,7 +121,12 @@ HRESULT CDecoder::CodeReal(ISequentialInStream *inStream, ISequentialOutStream *
     {
       _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;