coverity#983232 Resource leak

Change-Id: If4704847d4ef69d512f05dc12bcb01ac0d9d54b3
Reviewed-on: https://gerrit.libreoffice.org/2201Reviewed-by: Caolán McNamara <caolanm@redhat.com>
Tested-by: Caolán McNamara <caolanm@redhat.com>

vcl/source/gdi/pdfwriter_impl.cxx

@@ -2043,6 +2043,7 @@ void PDFWriterImpl::computeDocumentIdentifier( std::vector< sal_uInt8 >& o_rIden
             //the binary form of the doc id is needed for encryption stuff
             rtl_digest_getMD5( aDigest, &o_rIdentifier[0], 16 );
         }
+        rtl_digest_destroyMD5(aDigest);
     }
 }