ofz#4526 Null-reference READ with openssl crypto backend

which is not the default backend

Change-Id: I982e1b61c10efbe3312d68d90d48292a9b6cca0d
Reviewed-on: https://gerrit.libreoffice.org/45794Reviewed-by: Caolán McNamara <caolanm@redhat.com>
Tested-by: Caolán McNamara <caolanm@redhat.com>

oox/source/crypto/Standard2007Engine.cxx

@@ -124,6 +124,8 @@ bool Standard2007Engine::generateEncryptionKey(const OUString& password)
 {
     mKey.clear();
     mKey.resize(mInfo.header.keyBits / 8, 0);
+    if (mKey.empty())
+        return false;
 
     calculateEncryptionKey(password);