PPDCache: fix segfault due to access after delete

Regression introduced in:

    commit afe4d252
    Author: Noel Grandin <noel.grandin@collabora.co.uk>
    Date:   Wed Oct 18 09:43:21 2017 +0200

        use std::unique_ptr in PPDCache

Removing a naked pointer before inserting a possibly existing
one in a container is safe. This insured uniqueness (as the
comment suggests). However with unique_ptr, removal before
inserting deletes the pointer (when it exists), and the
insertion now taints the container with a wild pointer.

The fix is to skip adding if the pointer is already in the
container and add only when missing.

Change-Id: Ifc6b517451abb564949ccadfee10d98bf827540d
Reviewed-on: https://gerrit.libreoffice.org/44333Tested-by: Jenkins <ci@libreoffice.org>
Reviewed-by: Ashod Nakashian <ashnakash@gmail.com>

vcl/unx/generic/printer/ppdparser.cxx

@@ -576,15 +576,15 @@ const PPDParser* PPDParser::getParser( const OUString& rFile )
     if( pNewParser )
     {
         // this may actually be the SGENPRT parser,
-        // so ensure uniqueness here
-        rPPDCache.aAllParsers.erase(
-                std::remove_if(
+        // so ensure uniqueness here (but don't remove lest we delete us!)
+        if (std::find_if(
                     rPPDCache.aAllParsers.begin(),
                     rPPDCache.aAllParsers.end(),
-                    [pNewParser] (std::unique_ptr<PPDParser> const & x) { return x.get() == pNewParser; } ),
-                rPPDCache.aAllParsers.end());
-        // insert new parser to vector
-        rPPDCache.aAllParsers.emplace_back(pNewParser);
+                    [pNewParser] (std::unique_ptr<PPDParser> const & x) { return x.get() == pNewParser; } ) == rPPDCache.aAllParsers.end())
+        {
+            // insert new parser to vector
+            rPPDCache.aAllParsers.emplace_back(pNewParser);
+        }
     }
     return pNewParser;
 }