fix gif import crash

Change-Id: I3a5e0efef6843fb8f8b485fa2369ac5b99513c31
(cherry picked from commit 839e051c)
Reviewed-on: https://gerrit.libreoffice.org/16952Tested-by: Jenkins <ci@libreoffice.org>
Reviewed-by: David Tardon <dtardon@redhat.com>

vcl/source/filter/igif/decode.cxx

@@ -200,6 +200,8 @@ bool GIFLZWDecompressor::ProcessOneCode()
         pE = pTable + nCode;
         do
         {
+            if (pOutBufData == pOutBuf) //can't go back past start
+                return false;
             nOutBufDataLen++;
             *(--pOutBufData) = pE->nData;
             pE = pE->pPrev;