Accessibility: Avoid immediate crash in Impress

When Accessibility is active Impress immediately crashes. We need to check that
mpParent isn't NULL before we try to call it's methods. This has been a
problem since at least commit 7ef4a5bd.
I don't know if it is relevant to bring that back though.

Change-Id: Idaa1f30685205e3a62cec2a60a14791d798391a6
Reviewed-on: https://gerrit.libreoffice.org/8748Reviewed-by: Caolán McNamara <caolanm@redhat.com>
Tested-by: Caolán McNamara <caolanm@redhat.com>

svx/source/accessibility/AccessibleShape.cxx

@@ -402,15 +402,18 @@ uno::Reference<XAccessibleRelationSet> SAL_CALL
         throw (::com::sun::star::uno::RuntimeException, std::exception)
 {
     ::osl::MutexGuard aGuard (maMutex);
+    if (mpParent == NULL)
+        return uno::Reference<XAccessibleRelationSet>();
+
     ::utl::AccessibleRelationSetHelper* pRelationSet = new utl::AccessibleRelationSetHelper;
 
-    //this mxshape is the captioned shape, only for sw
+    //this mxshape is the captioned shape
     uno::Sequence< uno::Reference< uno::XInterface > > aSequence(1);
     aSequence[0] = mpParent->GetAccessibleCaption(mxShape);
     if(aSequence[0].get())
     {
         pRelationSet->AddRelation(
-            AccessibleRelation( AccessibleRelationType::DESCRIBED_BY, aSequence ) );
+                                  AccessibleRelation( AccessibleRelationType::DESCRIBED_BY, aSequence ) );
     }
     return uno::Reference<XAccessibleRelationSet>(pRelationSet);
 }